what about a black stomatella? Those have tiny little shells though, like half the size of your pinky fingernail. I can't see well enough from the photo.
i agree with the limpet it looks like a keyhole limpet. i havent seen them bother any corals before...........but keep an eye on it. also it could be a stomatella, black ones do have a black shell. however i believe that it is a limpet, a clearer pic will help.......
The link by chris and barb is EXACTLY WHAT THEY BOTH look like. Thank you all for the information. I see things like this and think oh no! oh my goodness crash or worse! Will we all die? Nobody ( the tank inhabitants) seem the least bit worried...I am the only one freaking out somewhat.
Thanks again.