Growing moss under terrarium conditions is extremely difficult. Most moss species need an extremely wet environment around them. This works for inhabitants of such environments like salamanders, newts and some frog species, but you need to remember that in those habitats reptiles are often rare. Or they are just from time to time there, nearly all of them need places to warm up too. This is no problem in the nature but regarding the size of typical terrariums it's no easy to provide both things.