I think it may have been a picture of the dagger, but it could be a fairy or a shadow. Whatever it was, it convinced me that Neal is Bae. Emma took her son being poisoned to make her believe. Neal just looked into a box to make him believe. He looked more resigned than shocked at whatever he saw. Emma was freaked out. So that makes me really think he is Bae.
I saw a lot of hurt in his eyes. It was almost as if whatever he saw brought back really bad memories. That’s why I’m also inclined to believe it is the picture of the dagger, He must have assumed the curse August was talking about had been cast by his father, and so it was his duty to set things straight.