Help with slug/snail ID please

[badAZZlars]

So tonight I have seen two of these slug/snails in my tank for the first time and it has been setup for nearly 4 months. I haven't added any new live rock since the tank was setup so it surprised me to all of a sudden see two of them.

Does anybody have an idea of the ID of these things? Is it a slug or a snail. It appears to have a small soft shell covering some of its body as viewed in the picture. I watched it for awhile and it seems to just be eating algae off the rocks but just was to make sure it won't be attacking my corals.

Thanks for your help.

http://farm6.static.flickr.com/5112/5837671595_e53754f8e3_z.jpg (http://www.flickr.com/photos/mike_larsen/5837671595/)
Untitled (http://www.flickr.com/photos/mike_larsen/5837671595/) by badAZZlars (http://www.flickr.com/people/mike_larsen/), on Flickr

[Mandosh]

Stomatella. Snail. Good.

[asylumdown]

They're Stomatella snails. They're good, they eat algae & detritus. If you try and scrape one off a rock, the back half of their tail will come off just like a lizard.

[badAZZlars]

Great thanks for the replies.