It's also way more mainstream than you might think or want to believe, with certain mainstream media outlets substituting its traditional racist and antisemitic underpinnings, including the use of terms of "white genocide" and "Jewish cabal," with a more politically focused narrative. Indeed, one in three Americans believe in "replacement theory."
How do you handle it if an employee complains that co-workers are promoting the Great Replacement?
As an employer you are potentially liable if you know or should know about racism in the workplace and do nothing about it. You have an affirmative obligation to take reasonable corrective action to stop the offensive misconduct from happening again. This does not mean that you fire any employees talking about the Great Replacement. While employers should have zero tolerance for racism in the workplace, there is a difference between an employee aping talking point he or she heard through a cable box and an employee going out of his or her way to intentionally offend a co-worker.
In the case of the former (and assuming an otherwise clean employment record), I'd recommend a warning and counseling. It's also a good reminder that you need to schedule some diversity training for all, which also serve to address and debunk this conspiracy theory.
In the case of the latter, you may have reached a crossroads in your employment that necessarily leads to termination.
I don't believe we solve problems by creating workplaces that are politically synchronous and racially homogeneous. In fact, placing everyone in their respective ideological, racial, and other corners likely makes things worse. We need to work (and live) with people who think and look differently. The "Great Replacement" offers an opportunity to engage employees around issues of race. Let's all try to make the best of this opportunity.