Relative to the Earth, yes; The inside of the airplane acts as its own kind of "world", in which the standard would be moving 500 mph relative to the Earth. As the fly is moving forward inside, you would add the 500 mph of the plane plus the speed of the fly relative to the plane to get the speed of the fly relative to the Earth.
Any motion is only measurable by comparing it with something else. In the same frame of reference in which the plane is traveling at 500 mph, yes, the fly is moving faster than that. But from the point of view of a passenger on the plane, no it isn't.
