I want to learn more about this system (but it's classified?). What would happen if there was some dirt on the lens? Would it appear to be an out of focus ball moving across the air?
Basically the MQ-9 uses something called the MTS, (multi-spectral targeting system):
These things (which are under most combat aircraft) are iterations of the same thing - a gigantic great camera (somewhere between 2500-3000mm focal length) which a whole load of additional image processing capabilities. They can do things like FLIR, detect rocket plumes, integrate with different systems (targeting, weapons, etc) but in the end it's essentially a gigantic great camera, on a very very smooth rotating fixture under the nose, or under the wing and of course, it costs $millions of dollars.
In combat situations, you can fly around all over the place and the camera can automatically lock onto a fixed point, and slew other sensors and weapons to the same thing at the touch of a button, so you can loiter around above a target and not lose sight of it.
Due to the focal lengths involved, anything on the lens wouldn't be rendered or visible at all, as with an ultra high focal length lens - the closest focusing distance is hundreds of meters, or even miles away - anything closer than that would be so blurred it would be invisible, you could wave your hand in front of it and you'd probably only see a slight darkening of the image.
Sensor dust would show up, because the sensor will render whatever light is being projected onto it (the image circle from the lens, which provided was in focus would be sharp) because sensor dust sits directly on the sensor - it would be visible.
However, it doesn't look like sensor dust to me, that does look like a real object floating stationary above the ground, and the illusion that it's moving, is actually caused by the fact the Reaper is flying at around 400kph and the camera is looking down on it, so it's basically parallax.
To give you an idea of what those of focal lengths can do, here's a video showing Lockheed Martin Sniper pod (used on F16s I think), observing targets 10 miles away;