An organic algae and mould cleaner product made from plant extracts should do the trick. Try looking at www.organiccatalogue.com Its an Organic gardening catalogue.
Theres a product i've used before called Algon. it's an organic moss cleaner and i found it works really well. I usually get through about a 15Ltr tub each year and one treatment usually lasts the yr. i bought mine from www.internetgardener.co.uk but i'm sure theres other places that also sell it.