"When moon?" is informal crypto slang, usually posted in a project's chat or social-media replies, asking when a token's price will soar. It comes from the phrase "to the moon", which is explained in full on its own page, and is often used mockingly about holders who care only about price.
It belongs to the same family as when Lambo, HODL and FOMO. Projects whose communities talk mostly in these terms are often treated with caution, as hype can precede a dump.