I notice a lot of people missing this quest because of Pharos Sirus first quest ending right when you unlock Pharos Sirus.

It is easly to miss since it spawns in Aleport after you complete Pharos Sirius once so anyone running Pharos Sirius through DF in another map will easily not notice it until they're given a reason to return to Aleport.

So if you miss this 2nd Pharos Sirus quest get back there now and get your free gil and a small cutscene providing a backstory to who the Pharos Sirus Zombies originally were, who were the Siren's first victom, and the Siren's origin.