hitchhicker that has fishingline?
 

We just got some new live rock and in it it has this thing that lets out this white string (looks like fish line) and it sucks it back in. what is this?and is it good or bad?We have about six of these.to small to take a pic.

Unfortunately without a pic, random guesses are all we can do. There are ALOT of critters, ineverts, even corals, that have small white strings like fishing line.

What does the critter look like?

Im going to guess they are SPIONID WORMS.
or possibley VERMETID SNAILS.

Check out this link from our Canreef Library on Hitch-Hikers.

http://www.xtalworld.com/Aquarium/hitchfaq.htm

Could be a vermitid snail
http://www.xtalworld.com/Aquarium/vermetid.jpg

I agree, sounds like vermitid snails. I have some too. The small ones aren't much to worry about but the bigger ones, depending on where they are, can irritate corals. I had one on the skeleton of my red open brain that caused some tissue damage from the string it shoots out and retracts (thats how they eat).

HTH

All I can see is the fishline

I have several of the same in my tank. The thread that they "cast" is thicker at the end, kinda fuzzy. Never seen the animal itself, the line just comes out of holes in the rock. All they do all day is cast, reel in, cast, reel in...

If it looks like a white thread,Goolge DIGITATE HYDROIDS. Maybe thats what they are.Heres a picture.Right beside where it says click on thumbnail

http://www.melevsreef.com/id/kritter1.html