It looks more like something was spilled on the film more than anything else. If there were any compositing problems in those shots there would be anomalies around the tauntaun or the Rebel transport, as everything else in the shot is "real". And I think we're all familar with what garbage mattes look like by now. Given the number of bugs that crept in when the video masters for the DC boxset were made, I can believe somebody was drinking in the telecine room and had a little accident!