Mirror magic is a complex, multi-faceted craft. The fey and alchemists use different tools to accomplish the same end goals (though both sides would be offended if one were to imply neither type of magic is more powerful than the other).
Moira, neither fey nor alchemist, is a rare breed: a necromancer. She commands power over life, death, and all forms of quicksilver magic. As a sort of personal joke, Moira has taken the nickname “The Iron Witch,” a reference to old wives’ tales about pieces of iron placed just inside one’s door would keep necromancers from entering one’s home.
It will not surprise you to learn that iron doesn’t do anything to necromancers. Or anyone else, for that matter. It is a solid, stable material for alchemical mirrors, though.