I fight this stuff every year and don't know what it is! If I vacuum it is gone and then I backwash the pump and 30 minutes later it returns! It like a fine dust. Dead algae maybe???? I have always attributed it to being just dust that has settled or residue from baking soda or borax that has settled. Then last year i decided it could be mustard algae? But I have kept the pool at shock level for 3 days. Yesterday was the first day we were able to use the pool. The pool was crystal clear and NOTHING on the bottom and then the kids stayed in for 4 hours and I forgot to backwash after they got out. a couple of hours later I went out to clean up around the pool and this stuff was all over the bottom. Any ideas??? I want it out! My Chlorine was still at shock level after they got out so I just don't have a clue.