"moiderer" is an old or dialect word meaning murderer as pronounced by some speakers of new york city english with the highly-stigmatized curl-coil vowel merger. If your teen uses it, they have most likely picked it up from a book, video game, period drama, or a regional grandparent rather than a modern peer group. There is no cause for concern here; the word is largely out of contemporary teen usage. If it appears in a school text, it is being studied for vocabulary or historical context. You can treat it like any other archaic vocabulary item.