Yikes I found something.......

[supersmile]

I just started up my 40 gallon saltwater tank and it has been running for over 3 weeks now. I have live sand, 40lbs live rock, chocolate chip star fish, 2 clowns, 1 royal gramma, cleaner shrimp, snails, hermits and a big crab. Just today I noticed this long black thing come out of my live rock. it is about 3 inches long and has beady eyes and a tail almost like a shrimp. I am doing the best I can to describe it as I only saw it once for a quick second and it went very quickly into the rock again. Any ideas about what it could be and what to do with it?? Is this thing harmful do you think?