I can suggest you what you can do.

You can walk up to him, stare at him, and say, not loud, only for him: Sir, I couldn't tell what you just said since apparently you intended it to yourself, but from that word I happened to have overheard, it was not a friendly one. And I think you look like a "Chinaman" too since you look like a human being. Are you not a human being?

The above is only a suggestion. If you think reporting someone's private murmuring is more effective, then you can try it too.