I think it's a baby snail.

[Joshua Agostoni]

So I just bought about 12 pounds of live rock to help balance my tank. I think I got another hitchhiker. The first one was a cerith snail big enough to easily I'd but this new little guys is a bit harder. I know the pic isn't the best but he's so small it was the best I could do. If anyone has any idea what he is I'd love to know.

 

[Mike N]

Collonista snails seem to be the most common roundish hitchhiker snails, could be one of those.