A white Racist is a white person who shows or feels discrimination or prejudice against people of other races,
or who believes that a particular race is superior to another. Key word being OR meaning not necessarily both.
White supremacism is a racist ideology based upon the belief that white people are superior in many ways to people of other races and that therefore white people should be dominant over other races.
A white supremacist is an advocate of the supremacy of a particular group, especially one determined by race or sex.
So I would say a white person can be a racist but not necessarily support the Ideology of Supremacist, But a supremacist is Racist.
Granted this might show my ignorance but in dissecting the definitions of each this is my conclusion of the actual meaning of the terms.