Why not some sort of bug flying close to the camera (probably closer to the camera than those people are)?
Or poossibly CGI as other shave said. However, if it was a bug, it may look odd in the video due to its relative speed and the compression and interlacing of the video file. Thigs moving through a video frame/motion within that frame can often look unnatural/fake due to video compression.
So maybe CGI, but CGI aside, it could easily be bug.
edit on 3/11/2018 by Soylent Green Is People because: (no reason given)