<a href=showthread.php?s=&postid=13261949#post13261949 target=_blank>Originally posted</a> by LeslieH
It's hard to tell from the picture if it's a sponge or a tunicate. In either case they're filter feeders &/or DOM absorbers. If it's taking over your tank in the absence of suitable grazers you'll need to take the rocks out of the tank & manually remove it. If you try to scrub or rip it off while the rocks in the tank you'll just spread it around even faster.