Mushrooms won't hurt your frogs and fungi in your terrarium is a normal thing that will come and go as part of the biological balance of the environment. While there are indeed poisonous mushrooms, your frogs aren't going to eat them, and while there are many pathological fungi, those that occur as part of the system won't harm your frogs, either. What you are describing may even be a part of the mushroom mycelium, but either way, I wouldn't worry about it. It would also be next to impossible to remove adequately. Even if you started all over, you would just get more fungi as the spores are air born and all around us. Normally non-pathogenic fungi can become opportunistic pathogens of immunocompromised animals, including humans, but for healthy individuals, there is little concern.
-----
Patty
Pahsimeroi, Idaho
4 D. auratus blue
3 D. galactonotus pumpkin orange splash back
5 D. imitator
4 D. leukomelas
4 D. pumilio Bastimentos
4 D. fantasticus pending