It would seem to not be good. Clams are photosynthetic feeders (zooxanthilae take light and convert it into sugars for the clam) as well as filter feeders. What kind of lighting do you have, and how far from the bulbs are the clams depth? Are you feeding phyto? Pulled in mantles can indicate irritation from tank mates, like any angels or heniochus (moorish idols) nipping at them.
Let us know what is in your tank, for a better answer.
Catherine