Looks like you have a couple or even several. I would take that rock out and any others you got with it and soak for 48 hours in fresh water in a bucket or two. Under no circumstance do you want even a piece of one left in your tank. Each piece can grow another worm and when stressed, they drop multiple pieces of themselves in order to insure their survival. They will decimate your livestock.
Just to clarify, these aren't actually true "Bobbit worms" that you see catching fish in the popular videos. Most of these guys are just scavengers and only the Eunice aphroditois (The Bobbit) are fish hunters.