Hi all, wish I was reporting better news but... whatever the black material/algae (if it is that) has returned in pretty much the same areas and new ones. It took a few days after stopping the SLAM but unfortunately it's back.
My test results are:
pH 7.5
FC 6
CC 0
CH 300
TA 140
CYA 45 (I stepped it up to 40 via PoolMath to see what FC level maintains before possibly taking it to 60 as per the Chlorine CYA chart).
I did an OCLT overnight and it comes out 0.5 loss (FC 6.5 midnight and 6 at 6am the morning).
There's no question the SLAM cleared up the water brilliantly, but part of the difficulty re whether the black material/algae was being killed was the pool cleaner sucks up whatever it passes each day, and I was brushing the pool daily also.
Is there any way I can better diagnose what the material is? Should I consider another SLAM with revised FC targets? (I held FC at 12 as per guidelines, my CYA was 25. The SLAM ran for some 9 days as well. I did mention seeing the black dust residue which happens when I brush the black material/algae away throughout.
I partly worry that perhaps this isn't algae. It doesn't appear to be mustard algae from what I read (especially since this appears in areas where there is direct sunlight). Could it in any way be residue from the gunite coming through? But it is seeking the same places and gets worse over time... like algae does..
Other question is, see the pics. In the first pic, see the little circles (much worse than appears in the pic)? This is how it's formed in some areas, as these little circles (single cell algae?) before getting more. In the second pic, it shows where the pool cleaner has run past and cleaned the edges but where the black material/algae is there where the cleaner doesn't often run past.