I think Susan Strong is Finn's mother if she is a human and she is keeping it a secret from him. She has blonde hair and a similar face to his. She is tan as well which could be a fact that she hasn't been underground for long. I think if she weren't, they would have revealed it in an episode. Since she is supposed to return in season 3, I think she will be in the two part episode Holly Jolly Secrets because it will be a secret that she is keeping from Finn and she might spend Christmas with him.