Green, Mustard, Black and Pink: A Field Guide to Pool Algae
Not all algae behaves the same. How to identify each type, why black algae is so stubborn, and the treatment plan that works for all of them.
Key takeaways
- Every algae type loses to sustained chlorine — the difference is how hard you have to brush.
- Mustard algae hides on swimsuits, toys and equipment; clean them too.
- Black "algae" is a cyanobacteria with roots in plaster — brush aggressively and hold chlorine longer.
- Pink slime is bacterial, not algae, and lives in hard-to-reach fittings.
“Algae” is a catch-all for several different organisms that show up in pools. They all have the same root cause — free chlorine that dropped too low for too long — but some are much more stubborn than others. Identify what you are fighting first, then use the right level of effort.
Green algae
What it looks like: cloudy green water, slimy green patches on walls and floors.
How stubborn: the easiest to kill. It floats freely and responds quickly to chlorine.
Fix: follow our green pool clean-up plan: shock to the level your CYA requires, brush daily, filter around the clock, and hold until chlorine stays stable overnight.
Mustard (yellow) algae
What it looks like: yellow-brown dust on shady walls and steps that brushes off easily — then comes right back.
How stubborn: moderately. It tolerates more chlorine than green algae and hides on anything that has been in the pool.
Fix: shock and hold, brush daily, and also clean everything that touched the water: pool toys, floats, brushes, the vacuum head and hose, and swimsuits (wash them). Otherwise it is reintroduced the next time someone swims.
Black algae
What it looks like: dark blue-black spots, often raised, mostly on rough plaster surfaces and in grout lines.
How stubborn: very. Black algae is actually a cyanobacteria with a protective outer layer and “roots” that grow into porous surfaces.
Fix:
- Brush the spots hard to break the protective layer. On plaster and pebble, a stainless steel brush works; on vinyl, fiberglass or painted pools, use a nylon brush only.
- On plaster only, rubbing the spots with a trichlor tablet (wear gloves) puts concentrated chlorine directly on the colony.
- Shock to the level your CYA requires and hold it longer than you think you need — often a week or more.
Pink slime and white water mold
What it looks like: pink or reddish slime in corners, around lights, ladders and fittings; or white, tissue-like clumps.
How stubborn: moderate. These are bacteria and fungi, not algae, and they protect themselves in biofilm.
Fix: shock and hold, brush and wipe every fitting, ladder bolt, light niche and skimmer throat, and clean or replace the filter media if the problem keeps returning.
Prevention is the same for all of them
- Keep free chlorine above 7.5% of your CYA, every day.
- Brush the whole pool weekly, especially shady and low-circulation spots.
- Run the pump long enough to turn over the water daily.
- Keep CYA in range — high stabilizer makes every algae type harder to beat.
Rough, porous, aging plaster gives algae places to anchor. Smooth, non-porous surfaces — tile, fiberglass and seamless coatings — are much easier to keep clean. If you are fighting black algae every season, it may be time to look at resurfacing options.
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