What means ,,Nyappy'' ?

Nyappy is the "logo" of the band. Miku-san 発言しました that Nyappy was created like "Happy" and unnyappy is "Unhappy", but with time, last 年 (2009), they 発言しました it was used for everything (answering the phone, bye, hello, how are you, etc.)

What Miku is doing with the hand is the "nyappy"

NYAPPY means hello または cool an cafe style