My sister-in-law and her boyfriend have a 125g with many incompatible fish. I'm worried about their jack dempseys, occasionally they go super pale on the upper half of their bodies. Is this stress or
Could be stress but they both look very healthy. Females will change their color substantially when preparing for a spawn, which is possible since you have a male and female there. Unless there are other issues like refusing food, excessive hiding, etc, I wouldn't worry about it.