Little thing is probably a pod. It could be a eunicid worm eating your mushrooms, i am pretty sure i have a Palola worm, about a foot long and 1/8" thick i only see it at night but i havent had any lost corals... If you have a bobbit worm its bad news. Try looking in your tank with a red flashlight at night they have really good eyesight and are fast.
Worm ID