Clearly, referring to somebody as a "nigger" as Candy does is a very offensive term that says a lot about the state of racism in the times that this novel was set. However, for me, to really answer this question you will want to read and analyse chapter 4 of the novel, which gives us a thorough description of Crooks' room and possessions. When Lennie enters, Crooks is very abrupt and abrasive, telling him that he has no right to come into his room. He says:
"I ain't wanted in the bunk house, and you ain't wanted in my room."
When Lennie asks him why, he states the two reasons he feels why he is not accepted by the others on the ranch:
"'Cause I'm black. They play cards in there, but I can't play because I'm black. They say I stink."
Thus Crooks is excluded because of racism - the colour of his skin. This means, like other characters, he does not have a place to belong or to call home, contributing to belonging and loneliness as key themes of the novel, but also adding a unique element because of the racism that, it could be argued, doubly disempowers him.
The language usd by Candy to describe the stable buck tells us that the relationships between blacks and whites in the 1930's was that there was a lot of racism, but they did not care, and if someone called them racist they would not know what it meant during the early ages.