My red sea bannerfish broke out with same symptom and died after a week.
Before, it got worse, it looked like lympho growth at some areas.
I had it for 3 years before it broke out. No symptoms before.
The red sea bannerfish I tried before all died of the same symptom.
Nothing works: tank transfer, copper, formalin, prazi, antibiotics.
I think this is some sort of virus or mycobacteria that breaks out when they get stressed.
There was another thread where a guy had pyramid butterflies with the same symptom. Only 1 survived out of a group. The survivor lost all the spots but is probably a carrier for life. I think its titled "ich vs brook"
He tried formalin and copper and nothing worked. He then tried antibiotics and the rest died except for the one fish. I personally think the antibiotics didn't work but the fish developed immunity.