Hmmm, it didn't appear to be a bivalve by the way it moved around. At first I thought it was some type of slug or flatworm until I got the light out and could actually see it's shell..looks more like a gastropod (slug) with a thing shell just behind its head. It also had tentacles/feelers sticking out of the sides of the shell in addition to the pair at the head, which you can sort of see in the first pic.
I should've been less lazy last night and just pulled it out of the tank to confirm an ID...he was in a precarious spot below a birdsnest frag...and right above my duncan. Pretty sure he would've ended up being duncan food if I tried yanking him off the rock yesterday.