That's very interesting. It's not moving with the background stars. However it could still be a camera artifact - see how there's a 'trail' behind Pluto's disc? In frames 4-7, the object you've identified is in the same position relative to the trail (off to the right if you are following a line outwards from Pluto along the trail).
I made an image to show what I mean. See frames 4-7; it's not an exact match but maybe it's a clue: http://i.imgur.com/TADflVd.jpg
I hadn't considered the rotation, because as you point out, it doesn't exactly match up. It's gaining distance from the center of image as it moves.
It could still be some kind of internal reflection artifact (even dust?) which varies in distance slightly, I guess. I'm pretty sure the team looking at these images would have gone over every object in depth, so it's unlikely I've spotted something they haven't :)
Its movement per frame seems too consistent and predictable to be an artifact (noise) of the camera.