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melski is right. Your Tang has a case of Black Ich. Which is bad, and could kill the fish very quickly.
I suggest reading this article because although it doesn't discuss Black Ich directly, black ich is just another form of the regular white ich that you see more commonly.
I have to jump in and explain something....Black spot or black ich is a flatworm infestation and all though it's not hard to irradicate it needs to be dealt with as soon as possible. Do a search on google for Black ich and it will explain it better then I can...