"How's the afternoon grandpa?"
Translation:Habari za mchana babu?
This is a great question. My answer is based only on what I have studied so far.
I have not seen a greeting with a "za" without an following object.
I have seen "Habari ya mchana" a greeting, with no person it's directed towards. I have seen "Habari za nyumbani" a question, with "nyumbani" as in "How's YOUR house" which would be a possessive. I have seen "Habari za mchana babu", in which the greeting word "mchana" (afternoon) is turned into a possessive for the grandfather as in "How is YOUR afternoon".
So far, I can say this much. I have been guessing greetings with "ya" and everything else I have used "za" and so far it has been correct.