Mala mía is a popular Spanish slang phrase that literally translates to "bad mine", but it's used to mean "my bad" or "my fault". It's a very casual and cool way to admit you've made a mistake.
In the song, Feid says "Mala mía, es que no sé si estás con alguien" (My bad, it's just that I don't know if you're with someone). He uses this phrase to apologize for calling his ex out of the blue, acknowledging that it might have been the wrong thing to do.
This song is basically the ultimate late-night call to an ex. Feid kicks things off by admitting he messed up in their relationship, singing, "Está bien, me he equivocado, yo también he tenido fallas" [Okay, I was wrong, I've had my faults too]. They're in different cities now, he's in Medellín and she's in Dallas, and he can't stop thinking about her. He's reminiscing about their physical connection and asks, "¿cuando vamos a vernos pa' tocarnos como en esos tiempos?" [When are we going to see each other to touch each other like in those times?].
He's feeling bold and a little sorry for himself at the same time, calling her up but then saying, "Mala mía, es que no sé si estás con alguien" [My bad, it's just that I don't know if you're with someone]. The song's main idea comes through in his confident, maybe even arrogant, claim that "No ha llegado alguien que como yo sepa chingarte" [No one has come along who knows how to f*ck you like me]. Ty Dolla $ign echoes this feeling of longing, asking her not to give his love away and hoping he's not too late. It's all about missing someone, regretting the past, and hoping a strong physical connection is enough to get them back.