Green, Black, and Mustard Algae: How to Tell Them Apart

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.

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 for the causes, and green pool DIY steps 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:

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 and cloudy water causes.

Next step

Read about algae treatment at Marina Pool Services. Call (817) 896-1120 or use the quote form to describe what you see.