Your issue is with the white caulking at the fiberglass seams and not the black caulking around the windows, right?? To remove the mold/mildew stains in the white caulk first get a supply of lint-free rags (white T-shirt material). Then cut the material into 4 X4 squares.
Using acetone, wet a square and make one even swipe on a short section of caulk. Do not rub back and forth or reuse the piece of rag as this will only smear the dirt.? Repeat this with a new piece of rag until the caulk is clean. It should not take more than a few swipes at most, then move on to the next short section.
It also works well on the sugar scoop rubber if your vessel has it around the edge of the sugar scoop.
