I'd say it was a pen EXCEPT for the fact that there is no visible clip to the cuff, so it's clearly not a pen that he simply has a habit of clipping to his cuff. I don't know what it is, but it definitely has no business being there and the only motive for it has to be something nefarious. So take your pick....a device attached to something to have a clever way of not wearing a hearing aid but still hear people tell him what to say, something medical, etc.

You could tell right at the start of the debate when Biden was talking that he was being told what to say. He kept tripping over his words and skipping words in sentences, as if someone was telling him what to say and he was trying to follow along but misheard some parts so just skipped them and kept saying the words he could hear.