I do have a moss tree. It's in a 40 gallon hex tank, which is HUGE in comparison. I do like the idea of a moss wall for you as a background. That would look cool, be great for the biology of the tank, not take up valuable real estate, and the RCS will LOVE it! I made a mesh moss cave once that was cool. I used the sandwich method.
Take two equally sized pieces of non-metallic window screen (mine is fiberglass), spread java moss or similar around on one piece, place the other piece on top and attach them together. I sewed mine together with fishing line and weighted the bottom with lead bullet weights (also used for fishing). hang it on your back wall and in a while, you won't even see the mesh... just a lovely wall of moss. It would probably cost you about $10-15 if you didn't have any of the components