Thanks for your help.

I didn't measure the CC, so I will do that and report back (I have Ben's test kit).

Since the pool is a SWG, I don't pay much attention to how much Chlorine is going in or what the demand is. I just "adjust" the SWG so that I have chlorine in the pool (it's set at 30% now, and I run the filter/SWG 8 hours a day).

My pool is not screened, so it gets direct FL sun, and the water temp this time of year is around 90 degrees, and I get a lot of afternoon rain. So it's a pretty tough environment for a pool. But oddly, even with all the rain, I can't seem to get the CYA under 90 (over the past couple of years).

The problem started because I failed to pay attention to the SWG (the "red" light was on due to low salt, and I didn't notice it until I saw the all green algae in the pool) after I cleared up the green algae on the sides, I was left with the black spots on the bottom and steps.