What I understand is: the reason why Fushiko have the mist because she feels too lonely and have no love. When Yuu touched her hand, her mist went away because she found a love. Unfortunately, it came back again when she started read the letter because she felt she doesn't deserve to have Yuu. She thought maybe it's better to leave her alone so that Yuu can enjoy her school life? That is why she said she will go back where she came from (I think she meant to go back to her old town) and when she turns 18, she will see her again. It means they will have sex to become one (if you know what I mean) and her mist will go away again. I don't think she have a supernatural or something. It's just there is a meaning behind the mist.
This is what I understand