Kanata, Fubuki, and Matsuri all imitate Houshou Marine's voice, and alongside the real one try to make Aqua guess who the real one is.
Additionally, the whole stream beforehand was PPTenshi imitating Coco's voice and calling all the Hololive members, up until she called Marine and deciding to imitate Marine's voice when calling here, which then led to the Aqua situation.