We started noticing small piles of what looked like sand or dirt on the bottom of our pool a week or two ago, although the solar cover stayed on and we weren’t swimming so didn’t pay much attention. Because we hadn’t been swimming, we inadvertently let our FC level drop this week but corrected it. Last night we finally got in and realized it was probably mustard algae. There is nothing on the sides of the pool, and it is not slimy. It is just piles that look like sand and dissipate when brushed. The water is completely clear. We started a SLAM last night and got our FC up to 27 (tested using test kit TF-100). Early this morning the FC was still 27. I brushed everywhere to dissipate the piles (although it all reaccumulated shortly thereafter). Around lunchtime the FC is still 27. I’m stumped. What do we do?