It could be a type of feather duster that lets off some mucus. Check at the top of this forum under Sticky: Hitchhiker FAQ for the Reef Aquarist. then look under live rock hitchhikers (i think page 3) and it is in there. HTH
65 gal reef
2 PC's 192W each , 20 gal sump/fuge
4 false clowns, 2 firefish, cardinal, midas blennie, toadstool, zoos, shrooms, trumpet coral, torch, pagoda, frogspawn, poylps, red open brain, devils hand, hermits, snails, 3 cleaner shrimp, coral banded and a emerald crab.