That was my first thought some sort of bivalve (clam or mussel) Its a little tough to tell from the picture but it looks like you can see the hinge?

Thanks to everyone with the great ideas on what this thing could be. I really not much help here..... I will do what BCOrchidGuy is suggesting and stake it out one of these nights and see if I can catch him coming out. I have never seen it out any farther than you all see it there. If I scare him he goes deep into the rock and it takes it a full day to come back out to this position again.

I would be a little more concerned about the crabs. They tend to eat everything. If you put red celophane over a little flashlight it shouldn't scare the little creatures in the tank. You would be amazed at what you can see after the lights go out.

I'd try to catch and ID the crabs. A lot of them can be nasty... I spent months trying to catch one that was eating my hammer corals.

These little crabs we have are lightning fast and are very skiddish. They are light grey in color......does anyone know? Do these small guys turn out to be quite large, or do they stay small. I can't even get a picture of one to post. I will have to keep an eye on them......as if they are coral eaters ---punt--- out they go. I am safe for a while as we are still cycling the tank.

It is easier if you watch them go into a rock then pull out the rock.
That's what I did this morning. Tweezers and a turkey baster had it out in 10 min. Check it out in: why is there a spider in my tank