Ellie moves back, and watches... weird shit happen. The reflection goes all fuzzy, and then it's just them again. On Athena's side, she might notice the mirror crack, shatter, and then... melt. The quicksilver-like solution drips before surging upward to reform into the mirror as it was, unharmed.

Ellie looks back at Athena, confused, before breathing on the glass again so she can spell WHAT THE F

Athena can probably guess the rest.