I'm a little bit late on this, but here is a summary of pixiv dict on sakurai's p-chama

When Momoka addresses people with an honorific, due to her slightly childish speech, "さま" (sama) becomes "ちゃま" (chama). That's how the nickname "Chama" came about.
Also, in The Idolmaster Cinderella Girls, some refer to the producer in charge of Momoka as "ちゃまP" (ChamaP). That's what we see here.
However, due to the maternal vibes that Momoka exudes, some fans also affectionately refer to her as "Mama."