This situation, if reported, is apt to get the supervisor some training. That's how the state and federal HR's tend to deal with these sorts of complaints. And as has been pointed out, the complainer will not ever hear about it, though they may notice some change in attitude or lack of cordial relations between them and the person they complained about. If that were to become pronounced, then it should also be reported, of course.
I honestly believe that I'd probably say directly to him that I didn't like to hear the term "slave." There's certainly not much percentage in going home and worrying about the statement. "Uncomfortable and threatening feelings" might be an extreme response for this comment, and might not. At least if you had addressed this with him directly, you'd have a better handle on this.
It may not have been badly intended. In truth, politically correct speech is changing so quickly that it's tough to keep up, even for the most well intentioned of people. And a recent visit to a president's home has awakened me to the term "enslaved people(s)" instead of the term "slaves," which these historical characters owned and we pretty much had to discuss. And after it was explained to me, I do think it's a more respectful form of address. However, being as I hadn't heard it, or had my awareness raised that "slave" was in any way anything but another noun, and was an offensive term, I don't think I can be accused of using it to offend or insult or demean anyone.