I may be wrong, it might not be a water droplet, but the most rational explanation would be that it is something on the window. Let's get
analytical.
It's impossible to tell how far away it is. There may be a false sense of depth because it looks like an object which you'd expect to be large. To
prove this, imagine it being a reasonable sized saucer craft. Feels like it's quite a distance away from the airplane, doesn't it?
Now imagine it's something small, placed on the window. Feels just like it is. So we can't really tell it's distance.
However, apparently he didn't see the object when he took the picture. If you're taking a picture out the window, you'd most likely be looking that
way aswell. If that thing actually was out there, it would not be hard to miss it. If that thing was on the window, it would most likely be ignored.
Conclusion: it's on the window.