Danmu (a.k.a., barrage videos or bullet comments) is a novel type of interactive video streaming, which displays instantaneous user comments flying across the screen during the video playback to better engage the users. However, such fancy experience brings a considerable burden to the battery of mobile user devices that have limited capacity. For example, WiFi testbed experiments show 15% to 35% increase in WiFi network energy consumption because of the large amount of additional network traffic for user comments. On the other hand, current network energy minimization methods adversely impact the Quality of Service (QoS) of Danmu users, because they put off the transmission and then delay the display of the user comments that should match with the timeline of the corresponding videos. In this paper, for the first time, a heuristic QoS-aware network energy optimization algorithm is proposed to reduce the WiFi network energy consumption while still maintaining the desired QoS of Danmu users. Comprehensive testbed experiments using an open-source Danmu streaming system and with real Danmu user traces indicate up to 28% WiFi network energy saving depending on different system, network, and user settings.