Just from looking at the bottom of the shell.I'm gonna say its a limpet of some sort.
Here a good link to all sorts of hitch hikers:http://chucksaddiction.com/
Definitely a limpet. They are usually harmless algae eaters, but have been known to eat SPS corals too. If you are planning on ever having SPS, you may want to remove it now, while you can.
It shouldn't go after anything else, they are good glass and rock cleaners. I have SPS in my tank, and I wouldn't remove one if I found one, just because it's usually rare that they go after corals.