It may have been a bristleworm epitoke. This is the reproductive form of some polychaete worms. At first it will be trailing a large worm like sperm sac, which will eventually empty into the water column (and feed your corals). The ones I have are ~ 3/4 to 1 inch for the worm portion and 3 inches for the reproductive portion. The one in the photo is substantially smaller, but has the same general appearance
Bert