Since it looks like he's sitting on the seat of your car, I'm gonna go out on a limb and say he's officially harmless. But I have no idea what it is.:D
I have a couple of those in my tank, they mostly stay on the algea patches. I was thinking they might have been small turbos that hitched on my rock, but wasnt 100%. Just been keeping an eye on them to make sure.
Biffer your right they are colonista snails, I looked them up they seem harmless but there can be a lot of them. I think a Wrasse will eat them if they become a problem. Thanks