As a white guy who used to play basketball with guys from all races a lot, I can say that I have been called 'white boy' by a black player one time that I can remember in a pickup game a few years ago. It didn't bother me at all at the time, and since then I never thought of what he said as some big racial slur at me. He was a really tall, and REALLY good player. I blocked him out one time and then hit a few shots on him, so I just considered it him being po'd that some short old guy made few plays against him.
So from my perspective, if that was all Harrell said, man just let it go, he apologized and Luka accepted. There are way way worse things that could have been said. I don't think Harrell is a racist, and I certainly don't think he needs to be suspended or fined or anything.