I used to live in Humboldt, almost right on the beach and we had mold all the flipping time - or well, the start of mold. The best cure was staying on top of it. Prevention is best because once mold hits it's hard to take care of. To fight mold you can make a homemade mold and mildew remover and use it. Fill a small reusable spray bottle with two cups of water and three drops of pure organic tea tree essential oil. Every once in a while spray down your walls with this solution and wipe dry. This mixture fights stains, but better yet the natural antispetic qualities of tea tree oil fight mold and mildew.
IF it’s too late and mold is rampant, you can try to get rid of it using more chemicals (which sucks, but you don’t want mold because of the health effects). Check out the
CDC guide to mold for more ideas.
American Clay has a mold resistant covering and their colors are gorgeous - it's not paint exactly, but it is eco-friendly clay covering and some independent tests show that
clay plaster does not allow for mold growth on the plaster surface.
The fan is a good idea. No matter what, even if you get super mold resistant paint, it's not going to be a match for excessive moisture. I'd use the fan, keep the door open when you shower, and wipe down the walls frequently with a non-toxic mold fighter.
You can look for eco-friendly paint at the following links...