I have the same exact worms you describe, and digged around trying to find out if they were harmless or not.
I asked around on a couple of reef forums regarding these, and everyone thinks they are spinoids, although it seems most spinoids/chaetopterid feed by extending two tentacles out to catch particulates from the water (
http://www.reefkeeping.com/issues/2003-05/rs/index.php).
I have watched these guy go to town after the lights go off, and I have never seen 2 tentacles come out of their burrows, or do the other things mentioned from the above article.
I am very curious to find out exactly what they are, but even with a decent pic, I'm sure its not an easy task.
But from what others tell me, the worst these worms can do is make a mess from the solid tubes they 'eject'.
I'll try and take a pic or a short vid of the feeding action if possible.