WHAT'S UP! is just a greeting not a question not asking anything just an aknowledgement/ greeting in passing and the only reply in that tone , you have to listen for the tone, is what's! up and keep it moving. Important listen for the tone.
walking down the street and see someone, eyes meet you both have things to do or nothing to do whatever the case you say, what's up! and you keep it moving. don't just say this in any situation you have to use your senses if you don't have any senses don't try it because if you aint feeling it or you convey it in the wrong way it could be misinscrewed in the wrong way that same what's up? could be a challange or a question in which case if you can't reach for your feelings or senses just say hi, that nuetralizes anything other then the greeting.