What could be eating my soft corals?

I have something in my tank that likes eating soft corals (specifically
toadstools) that are placed on/near a particular rock. That makes me
suspect something that lives in that rock.

See these two pics:
http://sswain.tripod.com/tank/softcoraldamage.jpg
http://sswain.tripod.com/tank/closeup.jpg

Typically, large chunks of the toadstool will suddenly disappear during the
day, or overnight. The damage tends to look more like large bites than small
nibbles too.

I'd suspect a nudibranch, but I'm certain there aren't any in the tank -
I'd expect to have seen one by now, yet I've never caught anything in the
act. I'd also expect something like a nudibranch to move around and eat my
corals no matter where I placed them.

The only other thing I can think of is my widow blenny, but that doesn't
make much sense either.

Can anyone offer some suggestions on what this might be, and how to stop it?

Thanks,
Stu