Limpets, probably some keyhole variety. I've got some that spawned in my tanks, too, and they're algae grazers. I don't notice a huge amount of algae missing, but they seem safe enough.
There are species that aren't, but I think most are reef safe, so I wouldn't try messing with them unless you notice them being a problem.
For whatever reason, I found a lot of the babies in my skimmer last time I cleaned it. In the sump of another tank, they seem to prefer to stay in the shade when this small, but the adults will wander around in the open.