i would just wash it off in really hot water for along time. ive heard scrubbing it in bleach really cleans it well (of course rinse it with clean water after) but i have never tried it out. as for making java moss grow on it, the best way is to get a black thread, and wrap it on there with that. a rubber band will work but more of a hassle, not to mention an eyesore. anyways, the black thread will rot away after a while and u wont have to worry about removing it (unlike a rubber band) and messing up the scape. it takes a couple weeks to actually grow onto the rock. good luck