Definitely lame. Lame on off hind, the one closest to you on the video. It looks like its coming from high up either stifle or pelvis area. I had one the same and was very hard to diagnose. Could just be a pulled hamstring or muscle in her back.
I agree also - white hind I think. The horse doesn't look like he is putting full weight on this leg and is not bringing the leg as far forward and also if you watch the horses head it goes down slightly when this leg is placed on the ground. My understanding is that you can sometimes tell hind lameness by the head action at the front though if it was a front limb it would go up when the lame leg hits the ground and would be more obvious and therefore it is difficult to judge hind limb lameness from the head action. If you trot up in a straight line you may be able to see the lame leg sitting higher at the hip. Hope this helps.