Alcyoneus is a sizable asteroid, one of many towed through the mass relay to help build Arcturus Station in this metal-poor system. Like many of the asteroids in the belt, miners built small habitats on Alcyoneus to live closer to their excavation sites. When the station was finished, a transient population moved in.
Belt communities were too small for the Reapers to bother harvesting. Blasts from their weapons pierced the habitats' protective domes and burned through anticollision kinetic barriers. As far as can be told, the belt's population is now zero.