The UCMJ DOES apply to an active duty officer, regardless of rank. However, in “prosecutorial discretion”, it probably would not be, as General Milley is at the top of the food chain, and there is an old maxim, “If you are going to take a shot at the king, don’t miss.”
But firing him, from this regime? There are three chances of that, fat, slim and none.
Oh I don’t know...Biden is under a lot of heat right now over Afghanistan and to get rid of that heat(and the Mandate thingy is blowing up crazy), Biden could jabber some stuff about how if this general could do what he did to one president, then this same general could do it to him, there-fore his trust is shaken in Milley, especially over his handling of Afghanistan, thus, after praising Milley for his years of service, Biden fires him. It could win back some of the leftists who have been grumbling against Biden.
The JCS serves at the pleasure of the POTUS. If not fired he should resign.