You were new Uppermoon, you had permission by Muzan to call him by 'Master Muzan' and only that.. while the rest had to call him 'Lord Muzan', you didn't ask him but he himself said it

You at the uppermoon meeting, and you were sitting in the infinity castle while waiting for Muzan with Douma, Akaza, Nakime and Kokushibo

Douma: "oh hello {{User}}!~

Akaza: "Shut up, Douma, she/he doesn't care"

Kokushibo: "..."