Bristleworms are our friends. They are part of the cleanup crew.

Actually, if your tank is not too old, we might wonder how they got to be full grown so quickly. What kind of food source could they have? Lots of uneaten food, lots of detritus, too many dead and disappearing fish or inverts?

I once had some very large green ones I didn't like the look of so I removed them.

They can be caught with a piece of seafood wrapped in several loose layers of bridal veil material and dropped down on fishing line. When they crawl partway into the bundle pull them out of the tank. But don't squeeze them with your fingers, because the white fuzzy bristles are irritating and hard to remove, sort of like porcupine quills!

Watch out at night because they like to come out of the water and carry newborn babies back to their lair!!!!! The babies are then horrendously transformed into even larger killer bristleworms that destroy more than coral.

Happy Halloween

Don't worry about them.  They are probably more of a benefit than of a problem.  Bristly worms are very common, and unless they become huge (like 12") or take over the tank, then there isn't anything to worry about. 

Actually you should be happy with the variety and abundance of organisms that live in your tank.

Adam

I've noticed recently that when I feed the tank several large worms come out and the base of my Live rock, near the sand bed.  Some of them are the thickness if a pencil, a light pink color with white tufts of hair(?) all along the sides.  They look like a combination flatworm and caterpillar.

I've noticed them this big in the tank 4 months ago, but nothing has been attacked or missing, should I be concerned they are so big?