That is a bristleworm, a very common hitchhiker on live rock.
Many will tell you this is a pest. If you touch it, your fingers will
agree that it's a pest. It is, however, NOT A PEST. Until this thing
gets as big around as a pencil, it is fine. They are actually a
formidable asset in cleaning up the carcases of livestock that
might die off in a back corner or behind your rock, where you
cannot easily reach. They do not bother your corals or fishes
unless they're dead. Several species of Wrasses, Copperbanded
Butterflyfish and Arrow Crabs are all suppose to eat bristleworms.
The coral banded shrimp are also said to eat the bristleworms.
I have seen this on occasion, but not in any abundance. There are
also special traps that can be purchased from most LFS. They are
inexpensive, and relatively simple to use. But again, your tank will
benefit from these scavengers. Their over-abundance in your
system could be an indication of overfeeding.
In the event you ever accidentally touch a bristleworm, your
fingers will look like this:
Carefully remove as many of the bristles as possible with a piece
of duct tape, then Soak your hands in vinegar or rubbing alcohol
for a few minutes and the remaining bristles will begin to desolve.
The lesson you will have learned the hard way is; Always wear gloves when reaching in you tank. :D