This happens when you don't have enough air circulation. It's a tricky tricky balance between humidity and circulation because the circulation can rob the humidity. I had this same problem in my vision cages because I tapped off to much of the venting. Now I'm struggling to keep my humidity's up. If you have stuff tapped up you have to open up something for more circulation. As for getting rid of the mold you have.

Heat and light are the key. Bake them for awhile maybe.