Accent, probably. brother is pronounced "nih-guh" and monday is pronounced "nih-gurr." The R is very prominent in the latter and completely absent in the former.
People in almost every country besides the United States pronounce brother and monday the same way. In those cases I take context instead of pronunciation.
All of my fellow african american peers hardly ever get offended when somebody who is white refers to them as 'brother' because 99% of the time it's in a non-tribal tone of companionship rather than inferiority. In terms of monday, i've only heard it once in my life from some kid in elementary school and he was suspended for a couple days when he used it.
I live in a very liberal/diverse city so people are really respectful towards most races, ethnicities and backgrounds