Mushishi EP21 - The Cotton Changeling

A lady named Aki discovered a green stain on her wedding gown, one year later she gave birth to a boy who, now grown up, has the same marks on his body. It turns out that the boy was originally born not as a human but as some shapeless green blob, which then turned into a human. Her husband asks for Ginko's help yet he is unable to save her son because it's the work of a "Watahaki".