Marine velvet will cause a golden sheen to the fish, and you should be able to see little bumbs onthe surface of the fish. When was the last introduction? Fish can be infected at a sub-clinical level--meaning, the have very few parasites and no distinguishing symptoms. If a stress event occurs that lowers their immune systems ability to resist the parasite, it can develop into a full blown infection. Since tangs are prone to parasites (IME), it's possible another fish brought the infection in, and the tang succumbed. His death could have reduced conditions enough to stress the other fish, as well.
I would pull the fish out and treat them with an anti-parasite treatment. Leave the tank without fish for 14+ days, and then return the treated fish to the main tank.