If an airplane is traveling at 500 mph and a fly is flying forward inside the airplane is the fly traveling faster than the airplane?

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.