Looks like some weird artifact. The original author deleted all his original tweets back in October, so I can't find the original; but he did retweet Stephen King's quote tweet of someone else's tweet of a screenshot of his original tweet (wow I hate that sentence) and that screenshot doesn't have this artifact in it.
They match up so exactly that I'm pretty convinced this has to have originally been the same image, just mangled by some sort of an uploader or something.