The moon forgot its mission.

It is about people's memory and truth, their own position and confusion, happiness in escape and Enron in return choice, innocence and dark world, and nostalgia for beauty.

The disappearance of the moon means the disappearance of beauty and faith. People are frantically looking for the moon and its substitutes, which shows that even if people don't realize it, they can't live without this intangible spiritual peace. The moon is in the hands of an innocent child, which shows that only inner purity can get those small but real happiness. Without the real moon, human beings with only substitutes will continue to degenerate, but with the moon, children as pure as before will become heterogeneous. His companions, teachers and mother all forced him to give up the moon. Most importantly, the moon has grown up and can no longer take him home. This actually implies that he has grown up and is no longer naive. So he chose to go with the moon.

This is just a fairy tale in itself. It is a very emotional thing written according to one's own feelings. Maybe Jimmy himself doesn't know how much to show. Maybe later generations found more connotations in the clues, but this may just be Jimmy's momentary inner tremor.

So I can only say my understanding, and more can only rely on my own experience.