What about emailing DJI with photos of both the above view and the other side of the piece and ask them if they can identify it and what you should do?
Is there black plastic on the controller or anything else that came with the drone that this could have come from?
If the joystick is of a similar size to the ones for the MM then the plastic is about 15mm wide, is that correct? If so have a look at the yaw pivot (above the camera), on the MM its the only thing I can see that is near that size from that area, does there appear to be a missing cover?