IME, clam usually doesn't attach if it's not happy in the spot one placed it on. It looks healthy though.
maybe its shy ?
perhaps too much light, what lighting are you using ?
In my opinion, if your clam stays in that particular spot, it will die of light starvation in medium or long term. Clams move for one reason or the other, but there is no scientific evidence to suggest that they seek a better place while on the move. It is possible that your clam just ended up in there and doesn't/cannot move to a better spot.
imo, it will move to a better lit spot, unless there's too much bottom flow
Mine decided to walk around for a few days after about a week of having him. I have leds 8" above the water, and first placed it about 12" down on the rock work. Not sure if it was the light or flow. But as he was moving one day i figured it was a chance to relocate him since it was unattached. I kept it in the same area just about 6" lower on a different rock. Light is obviously only slightly lower, and same with flow. Hes been reattached and happy for months and months.
Never thought my AI Hydras were to much for him, maybe it was to much flow. Either way im happy hes happy!
Im upgrading to a 35 gallon cube 20 deep... how do you like your hydra on your tank? What do you have it on?