are you sure its moss? It might be algea or slimy bacteria stuff. If its clear and looks like mucus, its bacteria. If its green or brown, its algea. Also, do you mean gravel, or actual rocks? If its bacteria slime, then it'd be a good idea to boil the rocks for a few mnutes before putting them in the tank. That'll kill all the bacteria that causes the mucus. If its algea, try rubbing it off the rocks. If it doesn't come off, try an algea pad.