What type of tank is he in, what is his diet been (and have you had past problems with him eating,) are you absolutley positive hes been eating, does your tank provide coraline or sponges if he hasnt been eating your offered food, does he have hiding spaces and open swimming room?
Moorish Idols are notorious for not doing well in aquariums (even in the most established and impressive of setups.) They are known to refuse food and suffer a slow death through starvation (which could be what is going on, however the symptoms seem to have come on rather fast.)
We would need to know more about our setup to offer a good option into what is going on.