Ahh! Thank you translator for all your hard work! I hope this can continue because its one of my favorites, despite the added language difficulty.
To jump into theory crafting, mercury does make some sense considering how weird the great brainsnatcher is portrayed. Unlike the other ghosts, its form is liquid-like with peculiar patterns. (for a long time I though it might be some sort of amoeba). Looking up inorganic mercury poisoning (which most commonly happens when swallowing mercury), its mentioned that it will travel through your bloodstream to attack your kidney and brain. I don't think its a full answer but I love how we are getting some realistic options and town history on how the great brainsnatcher may have come to be.