yellow tangs are usually pretty voracious eaters, i would def soak his nori in garlic and any flake food that should entice him to eat . what other fish do you have in your system?? is the yellow lethargic or does he swim near the top or bottom of the tank?? when he swims does he dash around randomly or swim with one pectoral fin?? are any of his fins tore up?? the red spots are they really red or goldish colored?? how long have you had this fish for and any new fish added recently?? pics??
start with the full story .... how you got the fish, what it looked like when bought,was it healthy looking before,how long your tank has been running.
without some more detail or pics its quite hard to say what it could be
sorry