Spent 6 weeks performing a SLAM in Feb/Mar. Since then pool #s have been spot on and the water crystal clear. Two days ago I noticed algae on two of the ladder steps and also on the walls of that corner of the pool. I was perplexed but I think I know the reason and am looking for confirmation. The concrete pad around the pool has turned black in spots especially at the same corner of the pool where the algae formed. Is that black in the concrete actually algae and maybe was introduced into the pool when it rained hard and drained/splashed into the pool?