How long has the damsel been in your tank?
I would say that if everything else is fine (since we don't have any tests to go on), that the domino just died. Fish do kick the bucket for reasons beyond our control. They aren't immortal in even the best of environments.
I would just make sure my parameters were okay, make absolutely sure there are no signs of stress in your other inhabitants, and maybe run some carbon. If you're really worried, perhaps a water change.
92-gallon corner tank, 100lbs of LR
, 140lbs of sand, 250watt 10,000K MH
, 110watts of actinic PCs, Mag 7 return, custom refugium, AquaC EV180 w/ Mag 5
Female lyretail anthias, eyelash blenny, tomato clown, saddleback clown, firefish goby, 2 sand-sifting stars