I've had my clam in the same spot, same holder for 5-6 months now. Mantel always fully open. But today it decided to release itself form the holder. Scared the heck out of me to look an see it laying on it's side. So I righted it and placed it once again on the holder. But I did look under it and saw nothing of it's foot. Can they retract them? Do I need to be concerned? It looks ok and the mantel is open. I don't see anything that would be irritating it.
Nothing has changed with my tank. Aside form me feeding more frozen food types. Parameters are all good. Same flow. Lighting still the same.