--- title: "Green, Black, and Mustard Algae: How to Tell Them Apart" description: "Learn how to tell green, mustard, and black pool algae apart by color, location, and texture, plus pink slime, which is not algae, and what each needs." url: https://marinapool.multisites.ai/post/green-black-mustard-algae-differences publisher: Marina Pool Services phone: +1-817-896-1120 quote: https://marinapool.multisites.ai/contact --- # Green, Black, and Mustard Algae: How to Tell Them Apart Green algae floats or coats surfaces and is easiest to kill. Mustard algae is yellow-brown, shows up in shade, and returns after brushing. Black algae forms dark raised spots on plaster and resists chlorine. Pink slime is bacteria, not algae. Identify the type first, because each needs a different plan. Water carePublished October 21, 20264 min read ## How do I tell the types of pool algae apart? Look at the color, where it grows, and how it feels. Each type has its own look and its own level of chlorine resistance. Pool Founder, a pool service software company, notes that treating mustard algae like green algae wastes time and product. The chart below uses its identification table. Type Color Where it grows Feel Chlorine resistance [Green algae](https://marinapool.multisites.ai/glossary/green-algae) Bright to dark green Floating in water, on walls and floor Slimy, slippery Low, easiest to kill [Mustard (yellow) algae](https://marinapool.multisites.ai/glossary/mustard-algae) Yellow-green to brown Shady walls, steps, corners Powdery, brushes off, returns Moderate [Black algae](https://marinapool.multisites.ai/glossary/black-algae) Dark blue-green to black spots Plaster, grout, concrete Raised dots with a hard cap High [Pink slime](https://marinapool.multisites.ai/glossary/pink-slime) Pink to reddish-orange Skimmer, PVC fittings, light niches Slimy film Moderate (bacteria, not algae) ## What does green algae look like? Green algae is the one most people see. It turns the water green and makes surfaces slippery. Pool Founder calls it the most common type and the easiest to kill with standard chlorine shocking. After a storm, a pool often goes this way fast. Read [why your pool turns green after a storm](https://marinapool.multisites.ai/post/why-pool-turns-green-after-a-storm) for the causes, and [green pool DIY steps](https://marinapool.multisites.ai/post/green-pool-diy-steps-and-when-to-call-a-pro) for the process. Severity runs from a light teal tint to a pond-like green. Hydra Pool Services, a North Texas company, grades it by how much of the pool you can see through. ## What does mustard algae look like? It looks like yellow or brown dust on the shady side of the pool. It hides behind ladders, on steps, and in low-circulation corners. Pool Founder says it brushes off easily but comes back within 24 to 48 hours if not treated well. Yellow algae is often mistaken for pollen or sand. The brush test helps: 1. Brush the spot. 2. If the material clouds and spreads in the water, it is likely algae. 3. If it floats on top and filters out, it is likely pollen. 4. If it settles right back down without clouding, it is likely sand. The source adds that spores can survive on anything that touched the pool water, such as brushes, nets, floats, and swimsuits. That is why mustard algae can reappear after treatment. Clean those items as the product label advises. ## What does black algae look like? Black algae shows up as small dark blue-green or black raised dots, usually on plaster walls and floors, and on grout or concrete. Pool Founder says the hard cap protects the organism from chlorine, so brushing through it matters. Its guide says a stainless steel brush is used on plaster or concrete only, never on vinyl, fiberglass, or painted surfaces, where it will scratch the finish. Roots can go deep into porous surfaces. Pool Founder says that if spots persist after several treatment cycles, a drain with an acid wash or resurfacing may be the only lasting fix. That is a pro-level decision. ## What is pink slime? Pink slime is not algae. Pool Founder says it is a bacterial biofilm. It appears as a pink or reddish-orange slimy film in the skimmer, around PVC fittings, in light niches, and along the waterline. It thrives where sanitizer is low and circulation is poor. If it keeps returning, check circulation and the chlorine feed, not just the color. ## Where do I look first on my own pool? Walk the pool in daylight and check the usual hiding places: 1. Shady walls and steps, for yellow dust. 2. Plaster walls and the floor, for small dark raised dots. 3. The skimmer throat, return fittings, and light niche, for pink film. 4. Behind ladders and in corners where water moves least. 5. The water itself, for green color that spreads through it. Take a photo of each spot. A clear picture helps a pro tell the type, and it shows whether a spot grows between visits. ## Do all types need a different fix? Mostly yes. Here is the pattern from the guides, in general terms: Type Key idea Common mistake Green Test, brush, raise chlorine per the label, run the pump, clean the filter Shocking without checking pH or stabilizer Mustard Treat everything that touched the water, brush thoroughly Skipping the accessories Black Break through the cap by brushing, then treat Skipping the brushing Pink slime Fix sanitizer and circulation Treating only what you can see Pool Founder gives dosing figures, and those vary by product and pool size. Do not copy numbers from a blog. Follow your product label. CDC also says chemicals can hurt people when mixed or handled without protection. Store them separately in their original containers, and never mix them. ## When should I get help? Call a pro if you suspect black algae, if yellow returns after a day, or if you cannot tell what you have. Also call if the filter keeps clogging or the equipment seems to be part of the problem. Marina Pool Services offers algae treatment, and its team also cleans filters since algae often sits in the filter too. For routine prevention, CDC points to regular testing of chlorine and pH, and weekly service handles that on a schedule. Pool Founder lists more prevention habits: brush walls, steps, and corners on a rotating schedule, run the pump enough hours in summer, and keep the filter clean so flow does not drop into dead zones. You can also read our [green pool cleanup guide](https://marinapool.multisites.ai/guides/green-pool-cleanup) and [cloudy water causes](https://marinapool.multisites.ai/post/cloudy-pool-water-causes-and-fixes). ## Next step Read about [algae treatment at Marina Pool Services](https://marinapool.multisites.ai/algae-treatment-grapevine-tx). Call (817) 896-1120 or use the [quote form](https://marinapool.multisites.ai/contact) to describe what you see. ## Frequently asked questions ### How do I tell mustard algae from pollen or sand? Brush the spot. Pool Founder says algae clouds up and disperses, while pollen floats and filters out, and sand settles right back down. Yellow algae also clings to walls and returns to the same spot within 24 to 48 hours after brushing. ### Is black algae really algae? Pool Founder describes it as a cyanobacterium that forms a hard protective cap over its roots, which shields it from chlorine. That is why it is the hardest to remove, and why brushing through the cap matters. Caps and roots go deeper into porous plaster. ### Is pink slime dangerous? Pool Founder says it is generally not dangerous to healthy swimmers, but it signals low sanitizer and poor circulation. If sanitizer is low enough for pink slime, the pool is also open to other germs. CDC says chlorine and pH are the first defense. ### Can the same pool have more than one type? Yes. A pool can show green water and also black spots in a corner. Identify what you see in each area, and treat the toughest type first. A pro can sort it out if you are not sure. ## Pool terms on this page - [Green pool cleanup](https://marinapool.multisites.ai/glossary/green-pool-cleanup) - [Mustard algae](https://marinapool.multisites.ai/glossary/mustard-algae) - [Cloudy water](https://marinapool.multisites.ai/glossary/cloudy-water) - [Black algae](https://marinapool.multisites.ai/glossary/black-algae) - [Green algae](https://marinapool.multisites.ai/glossary/green-algae) - [Light niche](https://marinapool.multisites.ai/glossary/light-niche) - [Resurfacing](https://marinapool.multisites.ai/glossary/resurfacing) - [Pink slime](https://marinapool.multisites.ai/glossary/pink-slime) - [Stabilizer](https://marinapool.multisites.ai/glossary/stabilizer) - [Acid wash](https://marinapool.multisites.ai/glossary/acid-wash) ## Sources 1. [Pool Founder: Pool Algae Types and Treatment Guide](https://www.poolfounder.com/algae-types-and-treatment-guide) 2. [CDC: Home Pool and Hot Tub Water Treatment and Testing](https://www.cdc.gov/healthy-swimming/about/home-pool-and-hot-tub-water-treatment-and-testing.html) 3. [CDC: Pool Chemical Safety](https://www.cdc.gov/healthy-swimming/toolkit/pool-chemical-safety.html) 4. [Hydra Pool Services: How to Fix a Green Pool Fast in Texas](https://www.hydrapoolservices.com/blogs/how-to-fix-green-pool-fast-texas) Written by Marina Pool Services and checked against the sources above on October 21, 2026. City rules and product guidance change, so check the linked page before you act. Follow product labels for any pool chemical. ## Keep reading - [Green Pool Cleanup and Pool Water ProblemsA pool turns green when algae grows because chlorine is too low or the water is out of balance. Do not swim in green water. To clear it, test the water, clean the filter, brush every surface, raise chlorine as the product label directs and run the pump. Call a pro if it does not clear.](https://marinapool.multisites.ai/guides/green-pool-cleanup) - [Green Pool: DIY Steps and When to Call a Pro in TexasTo clear a green pool, test the water, remove debris, clean the filter, brush every surface, raise chlorine as the product label directs, and run the pump around the clock until the water clears. Call a pro if the water stays green, you cannot see the bottom, or the filter keeps clogging.](https://marinapool.multisites.ai/post/green-pool-diy-steps-and-when-to-call-a-pro) - [Why Your Pool Turns Green After a Storm (and What to Do)Storms turn pools green because rain dilutes chlorine while adding debris, phosphates, and nitrogen that feed algae. In North Texas heat, a bloom can start in 24 to 48 hours once chlorine drops near zero. Test the water, clean out debris, and restore chlorine and pH per the product label.](https://marinapool.multisites.ai/post/why-pool-turns-green-after-a-storm) - [Cloudy Pool Water: 8 Causes and Fixes for North Texas PoolsCloudy pool water usually has one of eight causes: low chlorine, high pH, high alkalinity, high calcium hardness, high stabilizer, a dirty or underworked filter, debris after a storm, or early algae. Test first with a DPD kit, then fix the reading that is off.](https://marinapool.multisites.ai/post/cloudy-pool-water-causes-and-fixes) - [How to Clear a Green Pool, Step by StepTo clear a green pool, test the water, remove debris, clean the filter, brush every surface, raise chlorine to shock level, run the pump around the clock and keep brushing and testing. Light green water can clear in a day or two. Dark green can take a week. Follow product labels.](https://marinapool.multisites.ai/guides/how-to-clear-a-green-pool) - [Pool Filter Buyer's Guide: Cartridge, Sand and DESand filters are the simplest to run, cartridge filters trap finer debris and use less water, and DE filters give the clearest water but need the most work. Manufacturer figures put sand at about 20 microns, cartridge at about 10 and DE at 2 to 3. Pick based on upkeep you will actually do.](https://marinapool.multisites.ai/guides/pool-filter-buyers-guide) ## Need a hand with your pool? Call or text and tell us what your pool needs. Estimates are free for qualifying services, and same-day estimates are possible when the schedule allows. [Call (817) 896-1120](tel:+1-817-896-1120)[Get a quote](https://marinapool.multisites.ai/contact) ![A pool technician working on an equipment pad](https://marinapool.multisites.ai/_astro/technician-equipment-pad.BW5xZMNY_Zum1K5.webp)