Yellow, Black or Pink Algae? How to Identify and Treat It
Most pool algae is green, but not all of it. Yellow, black and pink growths each act differently, and some of them aren’t algae at all. Treating the wrong thing wastes chemicals and time. Here’s how to tell what’s in your above ground pool, and how to get rid of it without damaging your vinyl liner.
Many algae guides are written for concrete pools and call for wire brushes or pumice stones. Never use either on a vinyl liner. Use a soft nylon brush only, and pre-dissolve granular shock so it doesn’t sit on the liner and bleach it.
Step 1
Quick identification
| What you see | Likely cause |
|---|---|
| Green, cloudy water or slimy green walls | Green algae. See Green Pool After Winter? |
| Yellow-brown dust in shady spots that brushes off easily but comes right back | Mustard algae |
| Dark spots that won’t brush off | Rarely black algae on vinyl. Often a stain, or mold behind the liner. |
| Pink or reddish slime in corners, the skimmer or the ladder | Pink slime, a bacteria, not algae |
| White, tissue-like flakes floating or clinging to surfaces | White water mold, a fungus |
Before treating anything, test the water with a Taylor K-2005 kit. Low chlorine or high pH is behind almost every outbreak.
Step 2
Yellow (mustard) algae
Mustard algae looks like pollen or sand on the walls and floor, usually in the shade. It brushes away into a cloud, then settles back. It holds up well against normal chlorine levels and hitches a ride on swimsuits, toys and pool tools.
- Balance the water, especially pH.
- Brush every surface, including behind the ladder and inside the skimmer.
- Shock the pool with cal-hypo shock in the evening, and add an algaecide made for mustard algae, following the label.
- Put your brush, net and vacuum head in the pool during treatment so they’re treated too.
- Run the filter around the clock, then vacuum and clean the filter.
- Keep chlorine higher than normal for several days, and repeat the brushing and shock if it returns.
- Wash swimsuits and toys, so you don’t bring it back in.
Step 3
Black spots
True black algae roots into rough surfaces and is mostly a problem in concrete and plaster pools. On a smooth vinyl liner, dark spots are more often something else:
- Rub the spot with a nylon brush. If it smears or loosens, treat it like algae: brush hard, shock and run the filter.
- If it doesn’t move, it may be a metal or leaf stain on the surface of the vinyl.
- If it looks like it’s under the vinyl, it may be mold or mildew growing between the liner and the base. Chemicals in the water can’t reach it.
Stains that won’t lift, or growth behind the liner, often show up on older liners. See 7 Signs Your Above Ground Pool Liner Needs Replacing.
Step 4
Pink slime
Pink slime isn’t algae. It’s a bacteria that grows in low-flow spots: corners, the skimmer, ladder steps and return fittings.
- Scrub it off everywhere you see it, including the skimmer basket and the ladder.
- Shock the pool and keep chlorine steady at 2–4 ppm.
- Clean the filter thoroughly, since it can hide there.
- Watch the hard-to-reach spots for a few weeks.
Step 5
White water mold
White water mold is a fungus that looks like wet tissue paper or white flakes. It often appears alongside pink slime.
- Brush and shock the same way as pink slime.
- Deep-clean the filter. Rinse or replace the cartridge, or backwash and clean a sand filter. See Above Ground Pool Filtration Systems.
- Clean the hoses and skimmer, where it can keep growing.
Step 6
Keeping it from coming back
- Keep free chlorine at 2–4 ppm and pH at 7.2–7.6. See How Much Chlorine Should I Use?
- Brush twice a week, especially shady walls, corners and steps.
- Run the filter long enough every day, and clean it regularly.
- Shock after storms and heavy swim days.
- Close the pool clean in the fall, so spring starts clear. See Winterizing an Above Ground Pool in New England.
For your weekly routine, see our Pool Maintenance guide.
If your liner is stained, faded or growing something underneath, a new liner may be the fix. Liner replacements across Massachusetts and Rhode Island, and site visits are free.