I just want to ask if I should stop them when I see this happening. The white cat, Pickles, is older(4y) while the gray cat, Nu, is younger (just turned 1). They are pretty close to eachother and like spending time together and they dont fight about food or actually, they dont fight about anything unless Nu annoys his brother so much he gets smacked once or twice to get the message across.

Ok now the question. Usually I can tell when they are play fighting and let them be, and most of the time, it is a play fight, but as seen in the video Pickles has his ears slightly back and hissed once(no sound in the video, at least I hope so, as the TV was going on quite loudly). I have separated them when this has happened before, but both of them seek eachother out to continue. And thats exactly what is making me question if I should separate them when this happens