The idea is that every sign is a symbol: Marijuana, Smile (Sonreír), a cross which is a symbol for 1, and a skull, which in Spanish is called cráneo, a word beginning by a c, the 3rd letter of the alphabet. All together they make “MS13”.

I have no idea if this is things gangs do. And if it’s a gang tattoo, that doesn’t mean is a member (he may just like the gang aesthetics, people do weird things), and that’s not important, because guilty or not, he deserves a trial.