Is this mustard algae?


Well-known member
Aug 2, 2022
Pool Size
Liquid Chlorine
Hi all,
So after a heavy rain storm or anytime the water sits without anyone inside for more than a day or two, I start to see these black spots on the bottom of the pool. I've made sure the chemistry is properly balanced, FC always at or above 3PPM but never above 6PPM. The black spots do dissipate if I take my pool brush and brush over them into a cloud but I'm not entirely sure if it's mustard algae or anything else. I did shock the pool and brought up the FC to 10PPM on Sunday before the rain storm. On Monday I also vacuumed the bottom of the pool and made sure every single corner of cleaned and took my time to balance the water again. However, these black spots keep coming back! Is it dirt/grime from the rain or is it mustard algae or something else? I also added 1.5oz of HTH Algacide after vacuuming on Monday.

Any advice? Attached photos below;

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