bristle worms?

Even the really, really big ones will only eat dead things. My monster bristleworm will be all over a 1/2 silverside in less than a minute, but won't bother my coral, anemone, or seastar.

 

thanks for the good news! i sure am glad i signed up here. took awhile to realize it but i think i finally noticed how much money ive been wasting listening to some of the guys at the lfs..ill keep buying from them, but i wont be taking anymore advice. i was really close to tearing my tank apart and attempting to somehow remove them all. dodged that massive headache...thanks again,

 

Just to clarify, there are tons of varieties of "bristleworm", that's a very generic term. There are some worms that can do damage to corals/other tank inhabitants (I had one that would kill/eat snails) but they are more rare than the beneficial ones that just do cleanup duties on dead stuff.

There is a bunch more info in the ID This forum stickies, a bunch of pictures etc if you want to try and identify which type you have and whether they could potentially be a problem.