I set up a small dart frog terrarium. White fuzzy fungi has invaded. What can I do? (At day H:80%, T:78F, At night H:100% T:65F). Little small green spots on the surface of the dry wood as well.
You can use a diluted hydrogen peroxide, but if you already put creatures in there you probably want to just let it go away by itself. I had this happen in my new one, I used hydrogen peroxide which got rid of it, but it just came back elsewhere, eventually it just went away.
The layer of sphagnum has no benefit, and holds too much water for frogs' feet and plants' stems. It is best removed and replaced with a layer of leaf litter a few leaves deep over the entire substrate.
It is hard to see in the photo, but there may be other issues to address. On the right side of the viv there is a depression, and the false bottom doesn't extend all they way. Is there a purpose for that? Looks like a problem spot, where water will wick up and keep the substrate too wet.
The frogs would benefit from many, many more climbing opportunities -- they are avid climbers.
What is the ventilation? Is there a fan built into the top (I see a cord). If so, is it screened off for frog safety?
That looks like a 10g fish tank. If so, considering a larger enclosure would be a good idea, both for floorspace and for vertical climbing. An 18 x 18 x 18 viv is a tolerable minimum size for Dendrobates species.