ick starts in the mouth and gills first where it has easy access to the blood it feeds on.
It then shows on the body looking like white dust dots.
If your tang has stopped or slowed in eating...ick should be suspected.....and out he must come immediately to lessen the impact on him and the tank.
Blue tangs are very prone to this parasite....
Better safe than sorry in this case would be my guess....if wrong...he can go back in the dt....if right....we can know treat the problem with one of the three standard methods (assuming it was ick). Can not treat something you can't diagnos.