Wonder Worm

I went away for a week and came back. My beautiful blue and purple feather duster was nothing but an empty tube(or so i thought). My father said that he thought he saw my urchin eating him, but i moved a piece of LR trying to take the empty worm tube out of the tank, and it looks like he morphed completely. The tube is now long and running down the side of a piece of rock. He hasnt come out since iv been back. What the hell is goin on here? Should i leave him exposed to the light or cover him again with the LR like he was. As soon as i noticed him my brittle star came over and started to try and pry the worm off the rock.