They are limpets. Often this type is killed thinking that they are the relatives of the sps eating limpets called key hole limpet.... This 1 never tasted any of my acros
Phil right it's a Black Keyhole Limpet. I have 3 or 4 in my tank. They only come out at night. They're pretty harmless, they eat algae. They attach to rock and they can be very hard to pull off. You should see a white shell in the middle, when it closes up some.
1st one I saw was in Mike's tank.
I purchase mine from a reefer that had it for a year.
I had it about year & passed it on to a buddy.
He's still going strong, for about 3 years.