It is him flaring. It can be either showing aggression or showing off for a potential mate. Whatever you do, though, don't be tempted to put them together.
Plakatkhmer: they may be fine now, but it is a very unsuitable long term home and they will not live full, healthy lives in it. Although bettas are capable of living in small spaces, they still need room to exercise or their muscles atrophy. They also need heat. If you put them in a bigger tank with a heater (at least a gallon each) you will see a great change in their activity levels.