strange dome sitting on rock

I just discovered some round, piramide-like thing on one of my rocks. It is grey and white and seems to take the color off the underlying rock. It seems to move as it changes from place but i can't see it moving.
I included two pictures, and put a circle around the "thing".

The most important thing is knowing what it is and if it is a threat to my 50 gallon aqaurium

GM

 

Looks like what you have there is a limpet.

See: Limpet

Wikipedia: Limpet

 

2nd, looks like a little limpet. I never used to notice them, but now they fill my tank with life! ;D

 

thanks

Thanks guys,

Now I know what it is, it can keep doing it's work (eating algae). Surely it came with a piece a brain-coral that I picked up from the sea-bottom on my last dive-trip.

Greetings,

GM