I have had a fiberglass pool for three years now.
Around the perimeter, at the water line, I have a row of tile. This tile is attached to the pool and grouted in what appears to be white silicon caulking, or some pool friendly version of this.
I have black mold/algie in the grout/caulking around half the pool, and it seems to be getting worse each year. I have tried a bunch or different cleaning things, including bleach, but nothing seems to help.
The pool company has been no help.