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I like moss walls and I like rock backgrounds that you adhesive to the back of the tank. There are several YouTube videos on moss walls.
If you buy moss you could cover some small lava rocks and or small driftwood with moss. I never saw it done to rock slates. In essence you put the moss on the rock, cover it with netting, secure the netting and put it in the tank and it grows through the netting. ( http://www.littlegreencorner.com/howtotiemoss.html )