Ok, so I just had a revelation, I think. I believe I mentioned that I had a 500 gallon spa attached to the pool? Well, it's attached but only with overflow into the pool. I never actually tested the spa water, thinking it was all the same, and it would splash it with bleach or shock, but not test it. We'll I tested it. Ph unmeasurable high, like probably 9, and no detectable FCC even after a gallon of Clorox. I think it has been bleeding into the pool and acting as a feeder of algae, and I was just band aiding it instead of treating the source. So I hit it with acid and more bleach and will retest often. Jeez.