Oh, this was really wonderful. I have so many thoughts here, so I apologize if they are a bit disjointed. I loved the mood here. That Ginny saw the decay for what it was. I love that Snape knew it was her and allowed the change. I love how the relationship has potential without anything being forced. They could become great friends or lovers or possibly this is only one moment of accord. Beautiful.
Though the line that really sticks me is this: She was mildly surprised that he could dance, and a small part of her assumed that he had learned so that he could dance with his ghostly Lily. God, that's just so sad and painful. And so very Severus. *sniffle*