Since your clue is
exposure, that suggests that what looks like a rippling scuttlefish-like mantle is in fact caused by motion blur. In addition, I suspect there may be prolonged, quasi-multiple exposure in play as well. If true, that would make the actual object (much) smaller than it appears to be on the clip. The 400msec clip contains 4 frames, making for 100msec/frame. Motion blur means speeds in excess of that, but not knowing viewing angle and distance to object makes it hard to translate that in actual speed.
Anyway, tacit's guess the flying squirrel glides pretty much like a wingsuit pilot (except it's the other way around, of course), but the point is whether the edge of a squirrel's patagium or its steering paws move this fast during a glide. Although that's perhaps possible, I happen to think it's not likely, a.o. based on
this slo-mo video clip. The clip also shows the fairly static braking tail of the squirrel, which seems absent in the mystery clip.
So, if it's an 'animal' instead of flying debris, I figure winged insects are a better guess, and one that meets various requirements posed by the clip. But exactly which insect (other than a probably two-winged species) I can't tell, sorry.
