Susanna let two people in a injured mother and child, later on they both transformed into monsters, she retreated back into cherry's room where she killed the child monster but she hid and got stucked and survived only on water making her malnourised
The monster mc killed was the mother monster. And the piece of cloth mc saw was just ripped off when susanna was escaping
Other novel readers may correct me if i'm wrong i don't remember it clearly