This is a very interesting video. There definitely seems to be something there... but what? It is a dark, semi-transparent shape that roils around quickly, almost in a cloud-like manner. It is positioned behind some of the wood beams and in front of others, indicating that it is really there -- a figure of some substance.
Is it a ghost? The video description does not indicate whether or not there have been reported hauntings in the old building, but because they were doing an investigation there, perhaps we can assume there have been.
It does not appear to be merely a shadow, but we would have to know the complete layout of the attic space, position of windows, and any possible light sources to make a determination. (We must always be cautious; a survey of the space could reveal a logical source that we cannot see in the video: "Oh! That's what it is!")
Whether it is a ghost or not, this video might well have captured evidence of some kind of unknown phenomenon.