Deresa problem, fully shut for days

Have a 4" Deresa (3" when purchased 9 months ago). He was in the sand but a couple of tangs were pecking at his mantle. So I moved him on a spot in the rock work and they ignore him there. He has been fully open and appeared to be thriving for months.



Then noticed he was retracting the mantle to some extent. Next day he closed up fully and has been that way for several days now.



Params are good.



Wondering if he is deceased. He's fully shut, and I thought that when dead, the valves will actually open up some (relaxed muscle).



So, do they close up for periods of time? Should I be worried if tightly closed? I've lost a couple of clams in the past for various reasons but they always died with valves open.



Thanks for your thoughts.