Like other plane-born creatures, genasi are not typical inhabitants of Alm, but are likely to exist as individual created entities.
The Overseers—who created humans and mor—are an obvious source for such creations, only too happy to experiment with elemental infusion and discard any failures. Such a creature would likely have escaped during the Parosean insurrection, so they would probably look to the Three Kingdoms as their people.
Other possibilities include primal magic; in ages past, many people of Alm worshipped primal elemental beasts. Perhaps a one might have performed a ritual to bind themselves to such a power, forever imbuing their descendants with elemental force.
Perhaps also a summoned elemental servant has simply existed too long on the material plane, and started to develop a concept of “self” beyond a human-like shape and a set of instructions. Much like other extraplanar creatures, elementals can begin to “go native” after long-term exposure to “being”.