This is an excellent example of why you should never use Twitter to have a nuanced discussion.
It depends on your definition of racism, and most scholarly definitions are "power and prejudice." In other words, a white person who is in the majority has more power than a black person (think Silicon Valley[1]), so the actual impact of the prejudice has a greater magnitude that turns it into "racism."
The commonly-used definition of racism, though, is basically the same as prejudice.
[1] NOTE: If it is a black person in a position of power (CEO) exercising prejudice against white people, then it would also be classified as racism.