It is a fungus. Dart frog keepers have presumed that it is Leucocoprinus birnbaumii. It won't really hurt anything, and is very common especially in drier enclosures, such as those "bioactive" enclosures that are run dry enough that they do not need a drainage layer (if you're not removing water periodically from the drainage layer, then your enclosure does not need a drainage layer).
There's a fair amount of internet rumor circulating about this fungus, especially in but not limited to its growth on tree fern fiber (especially TF from New Zealand), and I don't find much in those rumors plausible or find them to agree with my experience and informal testing. You could remove the fungus, but there is no reason to do so, and if the species finds conditions there to its liking it will come back.
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