I have an in ground pool that was originally gunite. 13 years ago I had it fiberglassed. It held up very well until recently it developed some chipping of the surface. I had a company redo the surface. They essentially sanded the old surface and recoated the fiberglass. It looked fine for a while. Now I'm noticing that the walls of the pool seem to have a dusty gray coating. It is most predominant near the water returns. It does not brush off, but I can wipe it off with my hand. The chemistry has been checked and it's good. The pool is approximately 23000 gallons and I have a DE filter which I recently disassembled, cleaned and recharged. Can you offer any suggestions as to what the problem/solution might be? Thank you.