I have had this algae problem for some time now and can't seem to clear it. It is only located in 2 or 3 small locations and right at the water surface on my flagstone. I have used a steel wire brush on a drill and completely removed it, but it returned the next day. Apparently, it is embedded in the stone. I'm not sure what kind of algae this is. The water balance stays good, however lately cya is a little high. This line of algae stays there even when all values are near perfect.
Here's a photo from a spot in the spa. You can see how clear the water is (water line just below the green algae line). Notice how there is no algae on the stones next to this one.
Anyone have any ideas on what this is and how to get rid of it?