it's going to be difficult to keep your calcium and alk stable in that small a water volume if that clam gets happy and starts growing. they can take up an amazing amount of resources.
i would definitely look at water quality first. if you're using unfiltered (RODI filter) water from one of the Great Lakes, i wouldn't even want to know what kind of nasty is in that. add in to that a salt mix that is deficient in many of the elements that clams needs, and you have a recipe for trouble.
like mbingha said, your lighting is a paramount concern as well. Tridacna need strong lights.
and lastly, when they're that small, they are much more difficult to keep than when they're more established. they don't ship as well, and are much more easily stressed. judging by the size of that intake siphon, that is a seriously unhappy clam.
have you checked it closely for long term indicators of health, such as a good growth edge on the shell, and strong reaction to stimuli?
did you check the byssus closely and under the mantel near the shell edge for pyramid snails?
all the Tridacna clams should show good mantel extensions, most beyond the shell edge, he looks fairly contracted and stressed.
doesn't this mean it's not filter feeding?
these guys are not filter feeders. they get the bulk of their carbon through a symbiotic relationship with their zooxanthellae, and are able to absorb other nutrients like nitrates directly from the water.
