They sound like bristle worms. Some people say they are harmless, others
claim they can cause chaos. I think it depends on the specific type. They
usually eat tons of left-over food that fall into the rocks and cracks. I
keep the population low in my tank by using a trap. But I do have a few
smaller ones in there. They can grow to 3-4 feet which you DO NOT WANT!
You can buy or make a trap for them. You can get a trap from Petco for
around $9, or just search for Bristle worm traps on google.com. You can
also go the more natural way and get a Arrow Crab or Long Nose Hawkfish
which both will eat them and control their populations.

Do they look like this? http://melevsreef.com/id/bristleworm.html