That's interesting. I wonder if perhaps your lighting may have remained on for an extended period of time? Frankly, corals bleaching can be caused by almost anything. So long as your parameters are in check I would simply monitor the system and look for anything out of the ordinary. As a precaution, a fresh bag of carbon could help. Whenever I've had a mysterious issue I'd add fresh carbon to rule out any foreign containment.We did and nothing had changed unless it was fixed before we tested
You should place the frags of the survivors at or near the sand bed for a few days. Allow the corals to brown out and rebuild. Once youve seen positive growth you can place them in the desired location of your choice.
Good luck and please keep us posted.