I have an inground 16x32 vinyl pool. Water readings are:
FCL - 2.53
TCL - 2.65
PH - 7.2
ALK - 101
CH - 152
CYA - 22

The water temp is 88 and the water is very clear. My problem is that I have black marks on my fiberglass pool steps. They look like scuff marks you would get from a black sole sneaker. I cleaned off the steps with a Mr. Clean Magic Eraser and got all the marks off of them. Two hours later I see that they are coming back!!!

I'm trying to figure out what this is and how to get rid of it permanently.

Alan