What type of ground cover do you use?

I have a terrarium housing a whites tree frog and was wondering what type of ground cover most people use over their substrate. I purchased "tropical moss" from blackjungle a month or so ago, but it just doesn't seem to be taking all that well. I think it requires too high a humidity, which is leading to mold outbreaks in the tank. Quite frankly I can never get moss to really thrive in there for some reason. Maybe my water is too hard or the UVB bulb is not intense enough or whatever. Do you use moss, rock, river stones, leaf litter, bark etc or none at all?

I use rock covered with coco husk and 2" sphagam moss over that. 1/4 of floor over heat mat has siliconed green pebble(dime size) glued together because of impaction danger. And the rock seems to raidiate more heat from uth. I too had the same problem with that moss starting to mold. It sarted out kinda green and just got brown. I hate using sphagam moss, and cant wait for spring so I can fill bottom with Spanish moss. For cleaning purposes, I'm going to plant it over a plastic screen, so I can pull the whole thing out and clean under it.

My Whites have a base layer of Zilla Jungle Mix then coco fiber on top of that. Then the center section has a large piece of sheet moss. The rest of the enclosure has leaf litter scattered here and there.