I would say that the ghost is real. Why? Two reasons. First, because the presence of the witches in the play already lets us know that the play will include supernatural elements. Second, the stage directions call for the ghost to actually be there, and so the audience would see the ghost too.
Now, is that certain? No. It could still be a guilty conscience, but that's why I think it is real. People are haunted privately in some traditions, due to their guilt, and that's what I think happened here.