I don't know if this is the "preferred" method, but when I got mold on my snake's hide from the humidity, I used a bit of bleach. Luckily, it was just a small amount of mold, so I just put a bit of bleach onto a q-tip and rubbed it on the moldy spots, then left it out in the sun to bake. All the mold died, and my snake is fine. If you have a lot of mold, though, I wouldn't suggest this, because too much bleach is very very bad (obviously)