If the whole body is soft, medusa worm. If you want a frag, I'll trade you for them. I like to keep them in a QT tank. The only issue I ever had was one that was being picked on. Apparently they release some sort of toxin when predated upon. I had one in my main tank that like to stay in a certain spot. I noticed a wrasse picking at it and the next day it was clearly dead as was a decent little part of the monti that was close to it. Because there was a mass of white-ish slime, I took it for some chemical warfare and let it be and thing healed up just fine, except for the worm.
But I only QT my corals and would like to have a few just to keep things interesting.