Hi, our pool is only 1 year old, and I noticed this black spot in the deep end when it was first built, but seems like maybe it's a bit larger now. Dove down for a closer look, and it feels smooth and indented, like a spot about the size of a quarter with plaster missing? Is this something that needs investigation and dragging the builder in for warranty work? There was tons of plaster coming up in the cleaner for the first 6 months or so.