Super weird critter ID...

[Quinn]

So the other day, I notice a weird slug thing on the bottom of one of my rocks, in a crevace. Initially I thought it was just a sea hare, because I had one of them before, then I realized it was sticking out of a tube. Now today, it's still there, and obviously it lives in the tube. It is the colour of a sea hare, and has what appears to be the typical slug tentacles. The tube it's in is white and looks like what a feather duster would live in. It seems to be emitting a string of some type, almost like cobwebs, but underwater. It flails around sometimes like a snail on it's back.

Any ideas?