While trying to catch suspicious snails in our tank one night (more on that later), we spotted what we think is a Peanut Worm or Sipunculida. He was not in a position for us to be able to remove him for closer inspection, but we did get a good look at him for identification. Here is a picture of a Peanut Worm, unfortunately not from our tank, courtesy of BeakerBob at Reef Central Forums.
From our reading, this worm eats leftover food and detritus (waste). He is just another hitchhiker we have come across from our live rock. You can read the forum post and view more pictures we found of the Peanut Worm here.