My clam won't close until I am actually picking it up, so I think they just get used to their surroundings. At the LFS, people put their hands in the tanks all the time, things are added and removed, and people are always reaching over the lights, so they are constantly reacting. But in a controlled tank I think they know what is around them and know they aren't going to be harmed, so they don't respond anymore until physically touched.
mine did the samething.. at first it was very finicky.. but after a couple months in the tank and it getting used to me doing water changes and the fish moving all around it doesnt get finicky anymore. the only thing i would be concerned about if its not opening..then u have a problem.. but for now.. ur clam is fine..
Do you feed him mysis or Krill ? was checking out a beautiful Blue one at the LFS last week and they seem preety cool