I've dealt with this same thing, but in my freshwater fish. There is a solution that you can buy called Maroxy that works wonders for infections like this. You will have to QT of course.
When I had it on my fish, the infection wiped most of them out faster than I could get a proper diagnosis for it. Out of 13 fish, I managed to save 2... most of them had been with me for 8-10 years.
Maroxy is a pretty gentle medication and I havn't seen any adverse reactions to my freshwater tank in using it.