If you're reading this article: what's up, #+x$ Face.
During one of her HC S10 streams, Pearl surprised an Skizzleman unaware that she was streaming. Skizz initially confused Pearl for someone else and told her that he almost greeted her with a "What's up f… face!" A laughing Pearl had to remind Skizz that she was streaming, which caused him to fly away embarrassed.[1]
A few days later, Pearl, Skizz and Impulse streamed together at Magic Mountain. To tease Skizz, Pearl and Impulse reminded him that they were both still streaming. Skizz then flew out of indignation away, and Impulse shouted after him: "See you later [beep] face!".[2]
Since then "What's up [insert] face" or "Sup [insert] face" have been used by chat at any moment involving faces. For example, Pearl's terraforming of cliffs often results in calls of "Sup Cliff Face." Some chatters even use this as a greeting among each other by inserting the name of the chatter.