Hummm,
I don't think it was becuase it stretched for light but more likely a fish, crab/shrimp. If the clam doesn't continue to be stressed by that fish/crab/shrimp, it's prognosis should be just fine. It'll heal. I had a large Emperer Angel that took multiple bites (after 2 years of co-existing peacefully together) out of my 12" squamosa. After seperating the 2, the clam healed up nicely with no adverse effects.