Damsels often cause more problems than they are worth.
I cannot agree more. Damsels once comfortable in their "territory" will wear down and kill much larger fish. Even one can upset a harmonious community tank.
It is sometimes hard to get photos of fish, let alone close ups of possible infections. Most if not all infections SHOULD require all fish to be located to a quarantine tank that has no live rock etc and medicated there.
The reason for the quarantine tank (AKA "Q-tank") is that most medicines have adverse effects on other livestock....sometime corals, sometimes inverts....usually both. A treatment for Ich can and will kill all inverts.
Ich also requires that since the display tank cannot be treated that it remain "fallow" (without fish) for at least 8 weeks.
Another tidbit is that there are diseases that are specific to certain types of fish. Clowns, Damsels and chromis are susceptible to certain diseases.
Your photo is unclear, but it almost looks like a white patch near the eye and head of the fish. this may help with identification and treatment.
Good luck.