I did my annual scrubbing of my pool tile yesterday and what I thought was black scum from dirt/sunscreen/etc. was in fact algae. My pool has always been sparkling clear and I've never had cloudy or green water. I've never noticed any algae in my pool so I was surprised to see algae growing on my grout above the water line.
The stuff was growing on one side of the pool that gets the least circulation (the tanning ledge) and in the spa away from the spill over. The other three sides of the pool were clean and really didn't even need to be cleaned. The stuff looked black but when the bigger pieces broke off I could see the dark green fiberous algae. It looked similar to what sometimes grows on brick siding in high moisture areas. It was very hard and it took a stiff brush with some cleaner to get it off.
I have not been brushing my pool as often as I should so I assume this is the cause. I have always kept my FC in the range suggested in Pool School and the water always look great so I don't think it's a water chemistry issue.
I saw another thread on algae above the water line and there did not appear to be a clear answer as to how or why algae would grow above the water line.
Any thoughts? Should I be concerned? I plan to brush the pool and the water line tile more often (like I did the first year when I brushed it every week).