As far as I understood it, Cereleum is a physical fuel rich in aether, but it's not literally aether, hence it can be burned in an engine, especially those that power airships, which is what the Garleans use it for (as well as heating and lighting - presumedly the later is what it's main use in Eorzea is for).

Because it's so enriched with aether, overuse of it may possibly cause some kind of influence on the overall aetheric density of the landscape, but if there is a direct aetherial environmental impact for it's use, it's not really mentioned to my knowledge.