Guess that Thing!

[whatcaneyedo]

Like fishytime said, vermetid snail:

Snails That Worm Their Way Into Tanks
http://reefkeeping.com/issues/2005-01/rs/index.php

Generally only the smaller variety are considered a pest as they are more likely to proliferate all over the tank creating sharp little tubes on all of the hard surfaces that will cut your hand if you're not careful. Read the article and make your own informed decision. I have had both small and large types and I personally haven't had any problems with them.