I think we were lucky to even get THX on Laserdisc. I don't think they would have had a video master, so they probably made one from their archived print of the director's re-release. I agree with SilverWook's thought about it possibly being light coming in from the telecine. With it being in the corner I'd say maybe even leaking in from sprocket holes that were damaged. That used to happen on some of my 16mm student projects.
And the white fiend is...?