Not pinched mantle. Just looks a bit shy and not fully used to it's surroundings. When I first take then out of quarantine to the display, they react to the fish swimming above them for a few weeks before they get "used" to it and fully open.
There is no chance that this clam has too much light. However, they don't need as much as other kinds and you should be just fine with the 6 bulb fixture.
What is on the bottom of it's shell? Looks like another shell of some sort.