There is a different between being a racist and racism.
Being a racist, is you being a racist.
When a person of a certain race (or even a large group) use derogatory language towards someone of another race... it's being racist.
When an entire race is either sentenced to slavery or genocide... it's racism.
There is a fine line.
Once white people are forced into slavery or murdered for merely being white... then you can claim reverse racism if you want.
So, when a black guy calls a white guy a "cracker", yeah, it could be considered racist, but that one instance is so unimportant compared to what that black guy has faced in his life due to racism.
I hope this comes off the way I intend it to.