Nobody knows and as long as neither of the three men want to talk about it, we'll never find out.
According to Memory Alpha, Stewart didn't get along very well with Michael Down anyway. And I guess even Patrick Stewart is capable of saying something stupidly racist. Maybe being a Brit, he was also unfamiliar with American sensibilities regarding race. Who knows... as long as it was only a single incident over a seven-year period, I don't see a problem. People make mistakes sometimes.