Maybe a vermitid snail. They make a web and let it out in the water to catch food. Find out where the web starts and there should be a little tube sticking out of the rock.
Here is what they look like, though mine don't really bulid webs often.
No, they are harmless filter feeders. If you don't like the looks of them in your tank you can cut the tube in half and hope you chopped off their head.
Originally posted by drummafish
Are those webs common-I recently had one and my six-line wrasse got caught in it. Havent' seen him since???
wrasses are known to build a coccon when they sleep...thats probably what that was