this looks exactly like the mustard algae in our pool. When it dies it lightens. We’ve been fighting it all summer with super shocking and vacuuming to waste. Currently adjusting Ph high and Chlorine low to use Yellow Gone. After coming across this site we are also ordering different steps to replace our wedding cake steps. I read further and saw where you said it was fine sand. Is your issue resolved?I ordered the FAS part of the DPT test. I have the K2005. It should be here today. FC 2-4, PH 7.8, TA 100, CYA 75. We use unstabilized chlorine pucks in the skimmer basket. View attachment 511387View attachment 511386View attachment 511385