Springtails and Isopods will help, but it depends on the extent of mold. Larger frogs seem to be annoyed by the movement of small insects within their substrate. I believe it is because they are more sedentary and like minimal movement themselves. Dart frogs on the other hand are constantly in motion so the springtails and isos rarely crawl on them.

White mold will actually spore and then die off relatively quickly within a few days. Black mold and if seen in or on the substrate should be removed, tank cleaned and new substrate used.

How often are you changing your substrate and which type are you using?

Larger sedentary frogs' substrate with no false bottoms and no drainage system to remove urine and waste should be changed monthly. Turning/mixing the substrate around helps a little.