I'd start with a powerwasher to remove the "slime". When you get all of that off there will probably be a yellowish stain from the waterline down. There are a few different acids out there that will remove the stain and are safe for use on fiberglass and metals. I have used a product called "Zing" in the past. You will want to dillute it, and don't leave it on there very long or you CAN damage the finish on your fittings. Afterwards it's a good idea to rewax the hull to recondition it.
It's a SORRY-ass job. Have fun!