The reason why Deus chose 'Skull' as his codename was self-explanatory. Since he specialized in bone manipulation, what better way to demonstrate that than to have a mask with the theme of bones?
It was the same for Black's case where he chose his codename as Black while wearing a black mask.
Deus had thought before that having a skull mask would be cooler than having a bland-colored mask or animal mask. It was the main reason why he chose a skull mask instead of a grey-colored mask that was suggested to him by the senior members.
Putting the fact that Deus didn't wear his skull mask that signified his status as a senior member aside, he didn't mind that the receptionist didn't honor him since even Deus wouldn't know that a maskless member in front of him was a senior member if he was in the receptionist's shoes.
