Try this first by (Premier Login ripperproductions) Janitors

Get a good, powerful, grease-dissolving cleanser. (Simple Green, Purple Power, or something like that). Scrub the entire hull down really well. Then, compound it with an electric buffer and some cutting compound... You should get most of the mold and dirt off it that way. If it looks good, wax the crap out of it. Twice. Then wax it again. Gelcoat usually comes back to life fairly easily. If all of that fails, let me know......

Ripper® - Long Island Customs