Just started playing the game, and I'm very, very intrigued. I hope you don't mind me guessing, but is the story more symbolic and metaphorical than literal? Thinking back on it all, this story doesn't feel like the usual yandere story, but more so of a sort of 'empty nest' feeling after losing a family member.
Back in the beginning, MC is the only one who decides to check out the house. In other stories I've read/played before, the relatives would think of selling the house or staying there temporarily until they figure out what to do. But MC seems to be the only one who decided to stay in the house, and also seems to be the closest to the grandpa. So they can't bring themselves to let go of the memories they associated with their grandpa AND the house.
And then the fact that HE is called Mould and even agrees to be called that. I'm guessing that the house is actually abandoned and falling apart, what with the dust everywhere and the mold on the walls and ceiling. As for Mould moving around, maybe that's symbolizing how he (the actual mold) is literally everywhere, thus why he moves so quickly and claims to be around when NOBODY has, quote unquote, seen him?
The biggest tell has to be how he's described: the sickening smell, the cracking sound of his neck (maybe the sound of the rafters being old), how he sounded like a lump falling when he knelt (the mold dropping from the ceiling), and then how he kept the windows closed (not letting in fresh air and keeping the mold in). The MC might be hallucinating Mould due to mycotoxin poisoning, which looks even more plausible since Mould seems to represent black mold (black hair, black clothing), which is HEAVILY TOXIC and can even cause psychosis. The beginning of Chapter 2 might have confirmed it for me with the red marks on the MC's wrist, which could imply that the MC just got urticaria (hives) from direct contact with the spores (Mould touching them while they were asleep).
I could be overthinking all of this, but as I kept playing, all these points popping into my head, and suddenly the story makes so much more sense in a melancholic way. The MC believes that an intruder has entered the house, but actually it's more like they can't bring themselves to leave the house...thus the mold 'claiming' them in a way.
Feel free to correct me if I went overboard in theorizing. XD But this is beautiful. <3