I use leaf liter over my Eco Earth or Plantation Soil substrate. Agressive eaters on occasion miss crickets and end up grabbing a piece of moss and then swallow it since they have a hard time removing it from their mouth or just think its the cricket. The moss binds up in their stomach and can not pass. Sphagnum Moss is the biggest culprit I find for this occurring. Took me forever pulling and untwisting a 6 inch long piece from a WTF one time and thats something I don't want to do again.

Sheet moss, found as Christmas tree moss or tropical moss seems to work well but needs the right lighting to survive long term.
Since your tree frogs should be found on leaves, vines and branches, the more you provide for staying off the substrate the better anyway.

I sometimes use Sphagnum moss between the drainage layer and the top substrate to help retain some moisture.