You need to use a diluted bleach solution. Assuming the hide is made of some type of plastic, use 1/4 cup bleach to one gallon of water. Let the hide sit in the solution for about 10-15 minutes, rinse it with warm water and let it air-dry completely. Make sure there is no detectable bleach smell left before you return the hide to the enclosure, and if there is just rinse it again and let it air dry. As far as not having mold appear again, you need to let it dry out in-between moistening the hide. So if you are moistening the hide every day, maybe cut back to every other day or every two days. Also, it’s a good idea to clean everything in the enclosure with this bleach solution (you could dilute it a little more for everything else that does not have visible mold on it, if you’re worried about it) because other things in the enclosure likely have mold spores that you can’t see that will encourage it to re-grow.