Technically the moon is fragmented so it may not be the whole moon?
Maybe just chunks coming down or something else possibly within the moon?
Who knows, maybe the plot could involve Hamura or something. I mean Kagyua herself is just again the gedo mazu and Black Zetsu is sealed with her. Both are basically unreachable due to the fact that they are sealed within Kagyuas own dimension and no one but her had the power to access it in the first place.
Otherwise Sasuke could possibly summon the mazo back with his rinnegan, but I doubt he would do that after seeing Kagyua, even if he is a total nutcase.