They are probably eating microfauna. Mites and baby springtails.
I thought the same at first. However, I do not seed my vivs often at all(once in 6 months), and they generally lick the ground after a good misting. I have scoured the floor of the viv after watching them do this, and I see nothing... I know microfauna is called MICRO for a reason, but they are doing this primarily after a mist only. I thought perhaps it was a lack in mineral needs that was driving this behaviour. If the common opinion is that it IS in fact microfauna, I would feel much better. I would hate to think the frogs were deficient after I try very hard to keep supps that are fresh on hand and use them...
Thanks for the input, Adven2er!
JBear