The subject of this picture is the tiny snail in the dead center. It is a Collonista amakusaensis, which is quite a mouthful for such a tiny guy. It is about the size of a BB, and is hiding in the shade of an Astrea above. These tend to come in as hitchhikers, and breed quite easily. If you look at your tank late at night, you'll typically see lots of them once you know what you are looking for. They never get any larger, and seem to assist in eating some algae. In the next image, with some other inverts to get a sense of scale. Reef Safe.
And for size comparison again, collonista to the left, Astrea to the right.
Here's a picture with some other life you might happen to spot in your tank when looking for collonistas.