Can't tell what exactly from the pic, but it looks like a regular
tube worm... i have a good sized one in my tank, his opening is almost 1/4 around and he spits out this web of mucus that he catches junk on and then he reels it in and eats it. If thats what you have, the mucus the worm is casting out might be bugging your clam enough for it to stop opening up all the way.
Maybe give more info on your parameters- Sg, pH, calcium, nitrates, nitrites, temp, what lights you run and timing, and how old is your tank, how long have you had the clams, how often and what do you feed them....? stuff like that, cause if it isn't a worm, next step is to find out what is wrong.
