I have a very bad feeling it is the a** end of a fulgida worm.
Crapola! I hope either it's a large portion of one and it's now dead or it wasn't one to begin with. People who don't like bristle worms would LOVE to have them compared to this predator. I actually know of a tank that was torn down and re-started because of these worms.
Good luck Ian. Everything is looking AWESOME!!