I remember seeing this thing on one of those gardening shows about "planting" moss. I remember them putting moss, buttermilk, and an other ingredient or two into a blender, then smearing the mixture on a rock. A few weeks later or so, they had moss growing on the rock. I would assume the same thing would work in the terrarium, but I would do it before adding the frogs. If you're interested in this, try googling it, as this is all I can remember.