Lo más correcto, como dice Pedro sería ¿Qué pasa? También ¿qué tal? o ¿cómo va? Esto va de mas informal a mas formal

Hi Abde, ''que pasó'' is an expression that refers to the past like '' what happened'' and ''que pasa'' is for present, means ''what happen" . As Pedro said those are expression are quite informal and commonly use on speaking spanish.

In Spain we use "¿Qué pasa?" as a greeting, it's correct but VERY informal.

¿Qué pasó? means "What happened?" It's easier to say "¿Qué tal?". Simple and universal, it works as well in Spain and as in Latin America.

It depends on where you're using it. Some countries on Latin America seem to use it as a greeting much like people on the USA say "What's up!" but on other places it just means "What happened/What's happening?"