Sorry I should have been more clear. It's just above the water line in the pool. There are some cracks and rough spots that go from the water line to the deck. only about three inches. After some online searching I have learned that it is apparently called black algae. I guess I panicked a bit but what's disturbing is now I am not sure if my testing chemicals are giving me correct readings. I will now go have my water tested but it appears that I didn't have enough chlorine in the pool. Thanks for your response.