What is this THING!

[Hairytank]

I picked up a Toadstool and this "thing" was part of the live rock.
It has a hard curled shell with little barbs on it.
The critter that made the shell resembles a slug/snail, but obviously does not move around much....as it is attached to the live rock.

It spews out a stringy sticky "net" with the consistency of well..forgive me...mucus. You can see some of this in the photo. At first I thought whatever it was may be dying, but then is began to gobble up the "net" and of course anything within it. Rather disturbing to watch...

Although curious about it, I was thinking of removing it...

Anyone have any idea what this is and is it harmful?