It does sound like Black Ich but I wouldn't take it lightly. No, it doesn't kill as quickly as Ich [white spot], but in a closed system [aquaria] it proliferates rapidly and can become a serious problem.
The best thing you can do is formalin bathes for all infected fish. The parasite is easily seen on light fish [yellow tang] but it can certainly infect other fish as well and go unnoticed due to darker coloration of fish. I suggest using a magnifying glass and giving all your fish the once over for a few days to see if you see any other fish with the infection.
The procedure for formalin bathes is listed in the FAQ Thread.
Also, by chance, do you have a pic of the infected tang?