some mollusk and clams burrow and live in rocks. I am unsure what type that is, but I have one that looks the same. It has dug down into my rock and is impossible to get out, but from what I have heard, they live in the rock with just the top coming out. Mine opens about 1/8" or less, but has never moved. You pictures look like what I have in my tank, but mine is deeper in a rock and not visible without moving the rock. Just keep an eye on it, but if it looks like 2 shells closed together, I bet that is what it is. They are usually attached with a foot, similar to a clam, if you pull it out, it will die.