Looks like what Melev posted. I have a bunch of them. What bothers me about them is that they build their tubes on my nassarius snails and on the bottom of my corals on my sand bed.
That's normal, and nothing to worry about. You could have zillions of them and they wouldn't hurt your system. At some point, your fish, hermits, crabs, or other inverts will start mowing down the worms and you'll end up with a balanced population.