Morra is popular Mexican slang for "girl" or "chick". It's an informal and affectionate term, similar to how an English speaker might talk about "their girl".
In the song, Becky G sings, "Si aún fuera tu morra, ¿qué no haríamos?" (If I were still your girl, what wouldn't we do?). Using this casual slang word makes the memory feel more intimate and real, highlighting the pain of wondering what could have been if she was still his special person.
Ever had a breakup that just didn't make sense? Becky G has. In "CRIES IN SPANISH," she's looking back at a relationship and trying to figure out what went wrong. She remembers her ex telling her all these amazing things, like "Según yo era tu droga, todo para ti" (Apparently I was your drug, your everything) and "Según no vivías un día sin mí" (Apparently you couldn't live a day without me). But she keeps coming back to one simple, painful question: if all that was true, "Dime por qué no estás aquí" (Tell me why you're not here).
The whole song is full of this confusion. She's so hurt that she can't even finish a TV series they started together or go to their old Friday night spot. She wonders if he feels any of the same pain, or if he even has a heart. It's a really direct look at that moment when you're trying to move on but can't stop questioning if anything you had was real in the first place.