A racially insensitive comment could be considered harassment — and it would be unlawful harassment if putting up with such comments became a condition of continued employment (essentially, management was unwilling to put a stop to it) or if the conduct was severe or pervasive enough to create a work environment that a reasonable person would consider hostile.
Since you have a duty as an employer to stop unlawful harassment, we recommend that you investigate the alleged comment and, if you find evidence to support that your harassment or other conduct policies have been violated, discipline the employee who made it. Make sure that the punishment fits the crime. For instance, something that was, in fact, just insensitive may warrant a verbal warning, whereas calling someone by a racial slur may warrant a final warning or even termination, depending on the circumstances. Be sure to document your findings and any disciplinary actions taken.