It came with a small rock it had attached to. Maybe 1 inch piece of rock. It seems to just stay attached to that. It started to attach to where I set it, but apparently moved a little and just kept itself attached to the piece of rock it came with.The clam will turn itself to adjust for flow.
Keep in mind it could have been in decline before possession.
Did you inspect the clam threads?
It has not yet mounted itself?