unlike most fae, merfolk are not harmed by exposure to iron, and even enjoy working and handling it. they do, however, display a strong aversion to lead, and have a mild distaste for tin.
all we really know for sure is - fae don't do iron - merfolk don't do lead, cadmium or mercury - merfolk like iron-triad metals, copper, and silver - merfolk adore iridium - everyone loves titanium