Coming from someone who lives in the moldiest place on earth, I can tell you that for anything that "grows" on a boat bleach is your friend. I'm not talking about common dirt, I mean plant like stuff. Mold, mildew etc.
The trick is to use the weakest solution that will get the job done. Lay them out on the dock and soak them in a solution of soapy water mixed with bleach. If they are really moldy I'd start with about 4 ounces per gallon. Work up from there if needed. My guess is that 8 oz per gallon will do the trick, but again use as little as possible. You don't really have to scrub but you can if you want to.
We live over a swamp. I had some stuff growing on the screens on the back porch of our dirt house, that nothing would touch. I tried every commercial house cleaner I could find, vinegar, a pressure washer, you name it. I eventually used pool chlorine in a pump sprayer mixed about 70% chlorine to 30% water. That finally cleaned them.
Bleach is hard on the stuff around it though, so be careful.
