From what I remember, the evidence is that it's much worse than the moon being a monster. Instead, it's probably housing a portal that the Twelve Apostles are going to use to manifest in a more enduring way than their avatar bodies. The MC receives a vision during his first evolution trance of tendrils connecting the moon to the earth and the knowledge that "they" will come down from the moon. That's implicitly the Apostles because he says that only one through six will be coming, seven through twelve are already...something that we don't know, his vision gets cut off by Klaus attacking him.
We've also got mention of some upcoming event called the "Black Moon". When the MC touches the goop from the Third Apostle that he found in Ella's old school, he gets a vision of the Third telling him to prepare for the Black Moon's coming. We've also got Valravn's conversation with Syla, where he says that he and Ella have been making their own opposing preparations for the Black Moon. Presumably the Black Moon is the time when the Apostles use the moon to make their entrance into the world. Which means it would be so much worse than just fighting a monster the size of the moon, these things are full-on gods. Dealing with them manifesting is probably going to be the entire end game.