MeruPuri Vol 1

MeruPuri by Matsuri Hino. A Shojo manga rated Teen, published in English by Viz Media.

Airi Hoshina just wants an ordinary, happy life, until Aram, a young prince from a magical world, falls out of her mirror. Thanks to a curse, he turns from a little boy into a teenager whenever he's in the dark, and his only hope of getting home is closely tied to Airi.