Turning off the labels, aliens would probably assume that the world is naturally full of green stuff that is dealing with some strange grey infestation.
I think they would draw the correct conclusion, actually. I know it's popular to compare humans to mold or cancer or whatever these days, but this kind of thing is both unrealistic and insulting to the aliens, who by the definition of the scenario are at least as smart as we are, quite probably more.
But what is the actual source?