ICH, ICH ICH, put him in a QT and treat with Cupramine like RBU1 has suggested. Also, use Selcon on some Nori to boost his vitamin intake. You can put it directly into the water if he is not eating.
Again, just like RBU1 said your other tangs will get it next. By removing the Blue Powder, it will be less stress for all. Your Blue tang will die within a week or so if he is left in the main tank and not eating. Not to speak of an out break of ich for your other fish.
I have been there done that.
QT,QT,QT,QT.............................. Saves fish, money, and time.