Mala leche literally translates to "bad milk," but it's a common Spanish idiom that means "bad temper" or "bad luck." In this song, it's used to express a defiant attitude.
The singer, trying to win back his love, admits his rival might be more handsome but asserts, "yo tengo mas mala leche" (I have more bad temper/attitude). It's a memorable phrase that adds a touch of rebellious charm and shows a unique aspect of his personality.
Hombres G's "Vuelve A Mí" is a super relatable song about a guy who's totally heartbroken and desperate to get his ex back. He remembers how things went wrong, saying, "Recuerdo que las cosas / Se estaban poniendo mal" [I remember things / Were getting bad] and that they stopped talking. He even blames silly things like being short and not knowing how to break-dance!
He's clearly not doing well without her, admitting, "Ahora no como nada / Porque sin ti no puedo estar" [Now I don't eat anything / Because I can't be without you]. He's willing to do anything to win her back, even learning to break-dance! He begs her to call him and come back, promising, "Nos iremos a bailar" [We'll go dancing]. He's so down that he says, "Estoy sentado / Apoyado en la barra de un bar / Deprimido y amargado / Sin beber y sin hablar" [I'm sitting / Leaning on a bar counter / Depressed and bitter / Without drinking and without talking]. He can't believe she left him for someone else, thinking he's better because he has "más mala leche" [more bad temper] and is worth "quinientas veces" [five hundred times] more than the other guy. It's a funny, sad, and very honest look at trying to win back lost love!