Believe me you'd know, basically the skin starts to peel off.
You need to move this fish to a QT tank and begin lowering the salinity, hyposalinity is the best cure for brooklynella and even though it might not be what it has....it is very fast acting and you shouldn't take the cance. In addition to the hypo, you should treat with an antibiotic like maracyn or maracyn 2.
Brooklynella is contagious to other fish (technically), but I can't say that I've seen even one other species of fish (besides clowns) get it in the last 14 years. And I've seen plenty in the systems with clowns that were just ate up with brooklynella.
I have 2 percula's and they have a white cotton wool on their fins.....just a little but they both have a small lump at gill cover, looks white. still seem healthy though...eating well and active. Been treating with Myxazin