This appears to be some sort of a cloud, but the witnesses claim that it was moving at such high speed that they didn't see it while the photo was being taken. So, obviously, no ordinary cloud.
I had the same thought as the first commenter on the article site, which is that it's a glint of light caught on the arc of the propeller during the exposure.
If so, it's a great example of how ordinary photographs can fool people (even pilots in this case) into thinking there was something in the sky, when actually it was something quite ordinary near to the camera, or sometimes even reflected in glass. I think we always have to be extra skeptical when people say something appeared in a photo that they didn't see while they were there. In general, photos capture visible light, which is more or less exactly what we can see with our eyes. Cameras are also prone to lens flares and other optical aberrations, as well as now digital processing issues and artifacts. Like that whole flap about the supposed beam of light shooting out of that pyramid all came down to an issue with the iPhone. I don't doubt there are strange things going on in the world, but we also need to stay discerning and scientifically disciplined to find the truth that may be out there. And the fact that there are so many hoaxers around makes it that much more difficult.