White mold in a new terrarium is common and is natural. It will run its course and go away in a few days. Cork bark gets this white mold very quickly when it is first introduced into a humid environment. One thing you can do is add some Springtails to the terrarium. Them LOVE to eat mold and will help keep the tank clean. On top of eating mold they LOVE to eat frog poop and decaying leaf litter or plat leaves. Eating all that waste in turns makes the soil healthier for your plants and will make your soil last longer.