At first, this appears to be a contrail fragment. But watch what happens 40 seconds in.

For the first thirty five seconds, this appears to be an ordinary contrail fragment distorted by the wind or by aircraft manuever. But then at 40 seconds, something very strange happens that includes not only the transformation of the contrail into another form altogether, but the appearance of a small gray object moving downward near it. What may be happening in this video is unknown.