Do some searching for uronema marinum. I have no first hand experience with this disease, but from my memory, it's a protozoa parasite commonly infecting blue-green chromis damsels. Kind of nasty stuff if IRC. Formalin or freshwater dips can be used to treat the initial infestation with a course of antibiotics.
Vibrio looks similar making it difficult to diagnose without a scraping and microscope.