Mustard Alge? Not sure if we killed it....


New member
Jun 7, 2020
Pomona, NY
Hi! I am a new pool owner and we had a professional open the pool and start us off. We were maitaining the chemicals in the pool and at first assumed the brown lines in the pool were sand(not sure from where) and let the robot vacum get them. After a week or two I realized it must be something else and figured it was mustard alge based on the sandy look. We tried just brushing and shocking, but it came back in the same spots the next day. Then we tried yelllow out and shock. And we’ve kept the chlorine levels up for a couple days ans its been growing back in the same spots a but less but in a white or gray color. Our FC level is still very high, but I cannot tell if this change in color means anything, any thoughts??? (I was hoping it meant we killed it, but it keeps coming back)


TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
Mustard algae favors the shady side of the pool, if that helps identify it for you. It could just be plain algae.

"Shocking" tell us very little. What you think of as high chlorine, we may consider far too low for your CYA level just for daily levels, let alone killing algae.

The solution to either type of algae is the SLAM Process
If you prefer the movie version:

After the SLAM has passed, if you think it's mustard, then you go to mustard level for 24 hours.
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