Nope I thought wrong. I thought it was a cichlid. Now that I look though it is in reef. and now it is time to put my foot in my mouth.

My guess would be stomatella!

Stomatella is the most likely answer. They come in on live rock and reproduce in your aquarium. They look like miniature Abalone.

They are an excellent algae grazer. If you try to pick them up they will leave behind the end of their"foot" like a gecko drops it's tail. The biggest ones I've ever seen is about 2cm.