There are two signs that a clam is not doing well that I know of:
1) "pinched" or partially retracted mantle - meaning it does not put the mantle all the way out
2) when they actually start dying, their ability to close starts to fade: they are not responsive in closing when a fish or your hand passes over, or they never close completely
Since it looks like the mantle is not fully extended, and you report that it closes every night, I would say your problem is of the first variety. Sometimes this is due to predators like pyramidal snails. I can't quite tell, but there might some on the shell in the picture. They pretty much look like grains of rice. Manually removal from the clam, outside the tank, is the least stressful. They will pretty much brush off. You can do a fresh water dip, but that is extremely stressful to the clam, so it might die from that alone if not done exactly right. You may have to do the manual removal periodically. You should also check all of your cleaner snails (turbos, etc.) to see if they have any on them. Usually that is how they get into the tank.
For clam questions:
www.clamsdirect.com, click on forums. It now redirects you to a new set of forums that also have other stuff, but there are plenty of threads to look through on clams and the same clam experts to answer new questions.