Poa is more like "cool" - more informal and slangy than nzuri. It's a different register and it's the standard response to mambo. I'm translating below to give a sense of the formality rather than literal meaning.
Mambo (vipi)? - What's up? Poa. - I'm cool./Not much.
Habari (gani / za asubuhi / ...)? - How are you? Nzuri. - I'm fine./I'm well.
And then there's shikamoo which is extremely respectful to elders and apparently comes from an Arabic word and literally means "I touch your feet", and the reply to that is marahaba.
Yeah, I was repeating something I had seen somewhere else without checking it myself. :-( Once upon a time in Duolingo you could easily find your old posts ... but now you can't ... so I know I've got more incorrect rubbish I wrote out there before I knew better, and I don't know how to find it.
On the plus side, I'm pretty good with my weasel words ... like "apparently", so people should know not to trust me I guess.
I don't know if it is mentioned anywhere else in this course, or in any other comment section, but, apart from the slang greeting and response Mambo? Poa!, the verb kupoa is a perfectly good Swahili word meaning to become cool; also to become calm/quiet. TUKI: http://www.elimuyetu.co.tz/subjects/arts/swa-eng/p.html . Example: "Pakua chakula kabla hakijapoa" meaning "Serve the food before it cools/(gets cold)". Also after a bereavement -- as a response to Pole ya msiba "Sorry for your loss", the reply could be Tumekwisha poa "We have (already) calmed down." Can a native speaker confirm these two uses of "kupoa"?