是個妖邪 This Royal Father Is A Fiend
才下殿，往外正走，行者飛下帽頂兒，來在耳邊叫道：「師父，這國丈是個妖邪，國王受了妖氣。你先去驛中等齋，待老孫在這裡聽他消息。」三藏知會了，獨出朝門不題。才下殿，往外正走，行者飞下帽顶儿，来在耳边叫道：「师父，这国丈是个妖邪，国王受了妖气。你先去驿中等斋，待老孙在这里听他消息。」三藏知会了，独出朝门不题。
逐句解 Line by line
He had just come down from the hall and was walking out when Pilgrim flew down from the top of his hat, came to his ear and said, "Master, this Royal Father-in-Law is a fiend, and the king has taken in the fiend's influence."
The confirmation, and it comes from the one witness in the room who could see. 兒 in 頂兒 is a suffix rather than the word for a child, and takes ér like all the rest. 受了妖氣 — he has taken in the fiend's breath — is a medical rather than a moral judgement, and Wukong offers it as one: the king is not wicked, he is infected.
"Go on ahead to the posting house and wait for the meal there. Let old Monkey stay here and listen for news of him."
The instruction that puts Tripitaka in the posting house alone and Wukong in the throne room invisible. Everything in the next block turns on this division.
Tripitaka understood, and went out of the palace gate by himself — of which no more.
不題 — no more of this — is the storyteller's formula for dropping one thread and picking up another. The novel is telling the reader to keep their eye on the gnat.
Wukong has now seen what Sha Monk guessed at in the post house, and the chapter has taken nine hundred characters to get from the guess to the confirmation. It arrives in a whisper into an ear, in a corridor, from an insect.
The way he puts it about the king is the important thing. Not that the king is evil — that he has taken in the fiend's breath. It keeps the plan from the post house intact: catch the demon, show it to the king, and the man may still be curable. Wukong has been consistent about this from the first night.