Well I believe this is the reason quarantining is so important, especially in
SW tanks. The tank he shipped from might have had
ICH or it may just be from stress.
I would recommend SLOWLY raising th SG to about 1.023-1.024. And maybe turn the heater up a bit (depending what it's set on right now). I would say 83 or around there, if it is ich, unless you have other invertebrates that need a specific temperature.
This comes back to the whole thing about quarantining. I would recommend getting a 20 gallon tank and keeping the tang in isolation with a temperature of about 83-84, with a sponge or in tank filter. Of course, I have just about no experience with salt water lol. I would wait for some other advice too.