Our gunite plaster pool was completed last summer. On one side of the pool, there’s a stone wall 3 feet high. We closed the pool in the fall and covered it with a solid cover. The cover goes up and over the wall, so the wall is covered by the cover. Today we took the cover off, and on the side with the wall, under the tile line there are dark stains running down. Any idea what this is? Since it’s on the side of the pool with the stone wall, we assume it has something to do with either the stone or the grout between the stones. How do we treat this? We’re frustrated since the pool isn’t even a year old. 
