Killer underwater spider or ...?

[soapy]

Originally Posted by mrhasan

Probably vermatide snail.

http://www.reefkeeping.com/issues/2005-01/rs/index.php

See figure 5 in the above link.

I was thinking the same thing. If it was my rock I would pull it out and grind off any vermitids before putting it back. Also you can use a turkey baster to stir up detritus then any vermitids will let out a slime rope alerting you to their location so you can hammer those too. All supposing it is in fact vermitids.