So, it's on a snail and has obviously attached its foot to it. This will most likely be a case of pick which one will survive. You'd have to kill the snail to keep it in a good spot and then it can still move if it wanted to...to move the clam would mean messing with the foot and damaging that would kill it. Though really cool, I wouldn't count on it being much of anything but a really fancy snail shell.
Can you get a better picture?
You are probably right with the elongated shape of the shell. It isn't symmetrical enough to be a clam, as it looked in the first picture. I still wouldn't expect it to be a long term inhabitant of your tank and end up being a really fancy shell.