So that snake was some sort of kami or something. I wonder if they were involved in the disappearance of Muu's father. This story's a lot more serious than it first started out.
@Nixnaw Is it? I assumed that the snake (he or she) was trying to get Mutsumi to undo the 注連縄 to undo, presumably, any kind of passive protection or barrier that the tree offered the mountains, garden of Eden-style.
@red_bean_casserole No way to be sure. While white snakes tend to have a positive image in Asian culture, as there is so much yokai folklore in japan I'm not sure which one, this snake the author is basing off on. I took it as the snake lived near / came from the tree but your guess about the garden of eden theme might be possible as well.
The fact that the half-vampire kid was specified to be a Japanese vampire implies there are NON-Japanese vampires running around. That implies other folklore is up for speculation, so the Garden of Eden metaphors might be very, very literal here.