this. doesn't look like it actually hit anything.

Thanks for the sources. From the Fairbanks video I don't think it's a missile. It looks more like some dust, debris or trash moving across the field of view, because it gets significantly larger as it goes across the frame. The fact that it gets larger so quickly, gives a hint it is something close by, moving (falling) past the camera. At 6:50 it is just about to hit the ground close to the camera, as it's falling almost vertically down past the camera view.

Indeed if we continue to watch the footage past 6:50 (example 6:54) we see a bunch of these same black specks in the air.

Conclusion: it's part of the debris raining down already from the previous hit on the north tower.