The Knight of the Sky refers to the Straw Hats using the plural ‘you’ pronoun おぬしら/onushira. It stands out due to being decently respectful- almost everyone in this series uses either omae or anta (both of which are extremely casual) as their standard ‘you.’ 

The second thing that’s interesting is that nobody talks like this anymore- it’s antiquated language. There are a lot of Japanese pronouns that have fallen out of use/become more modernized over the years, and this is one of them. He likewise uses 我輩/wagahai, a similarly antiquated and unusual ‘I’ pronoun, when introducing himself. 

So basically, from the first line that the first Skypiean character we meet says, we know this guy is way out of the loop.

The word the Knight of the Sky uses to refer to people from below the clouds is 青海人/seikaijin. It literally means ‘blue ocean people’, which kind of calls back to how One Piece’s primary oceans are called the Blues.