Snail ID? Fuzzy domed shell with fissure?

[summer57]

I was surprised to see this snail on the tank wall, and have seen them only a couple of times. There are two & come out late at night - well after the lights are out. If I shine a flashlight on them -- such when I took these pix -- they move away very fast. I've had this tank for a few months, but didn't notice them until a couple of weeks ago.

They're about the size of a quarter, a purple/brown domed fuzzy shell with a fissure or crack down the centre. At first, we thought they looked like baby abalone or maybe some kind of limpet, but can't find anything online.

Any ideas?? And I hope they're safe algae grazers!

snail.jpg