Hmm...

I walked out to check my pool this noon. I have this stuff in the corners of my deep end. Water looks great, is holding CL, but I still have this stuff.

I plan to vacuum it to waste tonight, then shock if it comes back. I'm not totally convinced it's algae. It could just be massive amounts of pollen/dust, but I don't know.

If I get a chance, I'll try to get a sample and run the test Richard mentioned.