Bathrooms and showering areas are bad about harboring mold and keeping areas too humid. I agree with Killer about the photo, it's just not easy to tell by looking at it. If you're able to take a damp sponge and wipe the dust away from the leaves, then it might be mold...but it could also be any number of airborne contaminates. If your humidity is above 50%, I'd recommend getting it down quick. 25-30% is a good range, and helps to prevent lots of molds/rot from forming.