Have been dealing with this since last year. I have a dark green algae that grows at the water line, but only on the mortar between my stones. I use the trouble free method for maintaining my pool with liquid chlorine, and I am about to switch to salt but I would like to get this problem solved before I do the switch. Here are my last test results:
FC 8.5 I raise this to 10 when I add chlorine.
I've attached pics of the algae. Thanks in advance.