ONE Chromis....funny.
I agree with the opinions that it is likely an injury from some activity, mouth damage is fairly common from moving gravel /rocks, fighting, etc.
I do however want to express my concerns about using Stress Coat as a cure or solution to fish health problems.
Stress Coat is a water conditioner / dechlorinator, not a medication. For the most part I would say that it is not harmful to use Stress Coat but is it not a cure to fish ailments.
The claim that "adding electrolytes" heals is not entirely correct. Electrolytes largely play a roll in hydration regulation, muscle function, immunity and more. All of these functions performing properly is desirable and do play a roll in the overall health of an animal but "electrolytes" themselves are not a healing agent. Because a fish is injured does not automatically mean that they need their eletrolytes replaced, especially in saltwater fish.
Electrolytes is a fancy word for minerals, mostly salt. In fact, electrolyte depletion in salt water fish, although not impossible, is unlikely. Electrolytes are composed of many of the same trace elements found in out salt water mixes and supplements. Saltwater fish drink water, Assuming a person maintains their aquarium properly, the fish is ingesting these minerals and barring an organ malfunction, will not need it's electrolytes replenished.
My main point is that if a un knowing person read the statement that adding stress coat adds electrolytes and that electrolytes heals, this could imply to many that all they need to do is add stress coat when their fish are injured. This could lead to avoidable fish loss.
There are numerous, better routes to follow to cure tissue damage. Good environment and diet enriched with vitamin supplements are at the top of my list. In some cases meds are required, but that doesn't sound to be the case here though.
If I scrape my knee, I don't need gatorade to cure it
Joel