Stains coming back?

[fofa]

When we bought the house in 2001, we had to have the cement pond (literally, pond). One of the tasks was replastering. They did this. Now, I am getting stains (gray) on the bottom and sides of the pool. Since the metals in my water are zero, and no other causes can be determined, I was wondering if it is possible for the old "stuff" to be bleeding through from the back?
Any ideas on this?