Given that this faction will be working as spys and (probably) influencing the other factions, it makes me think that they are pulling at the threads of the other civilizations, and would be known as such by the few smart enough to see their meddling. Since they are alein in biology and appear only as ghostly visages then they may well have lifespans much longer than the other factions, so they would be patient enough and have the vision to make a move and watch it unfold for years at a time. All to place the civilisation they are influencing into their desired tapestry of the galaxy.