Methane flare. Liquid methane will turn to gas as it warms up, so they need to get rid of it somehow. Nitrogen and oxygen they can just vent. Generally methane is flared to prevent explosive gas buildup and (the worst of) greenhouse gas effects. Sometimes it's cooled back into a liquid, but that is apparently more effort than it's worth because flaring seems common practice.

Edit: just remembered the second stage is hydrogen. So it might be flaring that, or maybe the smaller flare off to the side is hydrogen.