This is a very interesting question! Although it depends on the context someone wrote, I would probably say 'お尻を出して馬鹿にする' or 'お尻を出して侮辱する.' It could just be me, though. 'お尻を出して馬鹿(ばか)にする' = to expose one's naked buttocks (in a public place) as a joke 'お尻を出して侮辱(ぶじょく)する' = to expose one's naked buttocks (in a public place) as an insult I hope I explained okay.

if what you mean by "moon someone" is one which is written in the following links, https://www.wikihow.com/Moon-Someone https://www.gaigo.ac.jp/english/591/ there is no equivalence in the Japanese language, because basically we don't do that. If I try to say it, it could be "我が尻喰らえ”. (But that's rather an old word)

I'd rather not say it here.

There IS a vulgar phrase which means the same as mooning, though. It is a combination of two nouns.

I can't either. As far as I know, there is no specific single verb which describes the action to moon.