its not hard to grow moss in a terrarium. nearly every dart frog habitat iv seen, moss is growing. in the week or two I have had my moss in my tank i can see some "runners" start sprouting from it. There is an abundance of moss out there (in the forest). It will grow back in a few months so it not like you are killing the ecosystem or killing some other native herps because you take some moss form the forest.
i also tried putting rocks on top the bed-a-beast and had no luck. thats the main reason i got some moss.