Never seen it before, I do not have a picture as of yet.
But, it looks like a really small version of Jeffery, the work in Men In Black II. Without the cute little flower on his head.
I have seen about 1 to 1 1/2 inches of it. Segmented like an earthwork, but it acts like a snake.
Comes out from under the liverock, and its head literally dives at the substrate.
I will try and get an image today, but this is the 1st time I have seen him, and I am reluctant to stick my hand in the tank and move the rock, with a lionfish being the primary resident and all..
P.S. I have seen a /fireworm/ before, and it does not look quite the same. No legs that I could see on the segments.