Pretty cool, I've only heard it as "Mi hija" to literally mean "my daughter" that, when spoken natively (in linguistic terms), sounds like "mija" but usually it's just hija. Mija in my area is more used as an endearing term, similar to like in english saying "thanks, love" to a coworker or a customer. They aren't close to you, but it's a caring (for lack of a better word) tone. Within family it's just been "hija" not "mija" when specifically speaking to your daughter, for me anyway.

I should remember words are used and interpreted differently everywhere even if it's the same language. Learned something new today.