1. What does it mean?
생파 shortens 생일 파티. It names a celebration rather than the act of congratulating someone, and may refer to anything from a small meal to a bigger gathering.
Also written or said as saengpa
2. The nuance
생파하자 proposes a party; 생축 simply says happy birthday. The word does not imply a particular age, venue, or amount of money spent.
3. Examples
“올해 생파는 집에서 보드게임 하면서 할래.”
I'd like to have my birthday party at home with board games this year.
“생파에 늦게 가도 케이크 자를 때는 도착할게.”
I'll be late to the party, but I'll arrive in time to cut the cake.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.