preventative maintenance activity that is required now and then.
The caulking has to come off. since the enclosure is fiberglass you do not want to use the normal tools as they will gouge or scrape the fiber.
I would carefully find tails of the silicon and pull it off. It will come off in strands like rubber bands. Remove all the old caulk, then soak with bleach to kill all the mildew. Then completely dry and re caulk with 100% silicon caulk and then let it dry for two days without water hitting it.