[Prefect hasn't spoken to Willow much, but between their first conversation, and what he's seen of the emotional pain that comes with being given a soul after years of evil deeds, he's been kind of keeping an eye on her from afar.

He noticed her post earlier, and that is perhaps why he feels compelled to say something when he passes her, in the corridor. Even if it starts out as something as small as:]

Good afternoon, Sister. How are you feeling today?