It doesn't have a proper source, but...

"Ceruleum is an energy source in Final Fantasy XIV. Ceruleum is a refined form of Aether, obtained by draining elemental Crystals of their energy, usually in processing factories. It is a powerful yet highly unstable source of energy, mainly used by Garlean Empire to power Magitek engines."

Final Fantasy Wiki - Ceruleum

So yeah, ceruleum is basically liquid aether extracted from crystals.