I have attached a photo of something growing on my concrete pool. It is soft, and a dark green color, although photo makes it look almost black. It grows generally on the north side of the pool, starting on the steps. My water chemistry has been good throughout the 6 years I have had the pool.
I can scrape it off with a stiff brush. But then it re-seeds nearby when I scrub it. It is under the water, not above the water line.
Is this moss, or some type of clumped up algae? I have had this for 3-4 years, but it's getting worse this year.
Any help would be greatly appreciated.