The problem is the caulk in the joints regularly become moldy. All the regular methods of cleaning only lasts a short time and the mold returns. I think my best attack would be to remove and replace the old dried out caulk. Problem is how to get the old hard caulk out without major damage to the fiberglass. The joints are pretty small, 1/8 inch or less, so there is not much work room.
Before I go at it with my utility knife, I thought I'd see if anyone had any better ideas.