The moon kanji is made up of the moon radical.
Since げつ and がつ are quite similar, we'll use one mnemonic to cover both of them. I expect your experience learning Japanese throughout the future will help you to memorize and differentiate between the two. When the moon goes down and goes away, everyone in the world gets (げつ) up, because that means it's going to be daytime soon!
Currently there are no similar looking kanji which exist in the WaniKani learning system