Hi chem geek! Thanks for your time. This stuff is on the bottom, scattered in slimy looking clumps across the pool here and there and in seams too. Yes it sweeps into a brown cloud of "dust" when brushed and reappears several hours later or the following day. It was yellow/tan yesterday in the filter and was spongy(much worse than shown in the pic). I drained my pool about a month ago because when I took the cover off, I still had some streaks and clumps on bottom. I then scrubbed and bleached(diluted) the inside of the entire pool. I couldn't believe it when I refilled and still had this "stuff". I'm adding a couple inches water before I test today. Curious to see what my numbers REALLY are!!!