Um, it's not illegal and it's not affecting their job.
WHY BOTHER?
What is your position? Are you managing this person? Why is it gross? If it really, really makes you uncomfortable, talk to the SM, and have them deal it.
As a SM, I might say something casually about it to someone, if someone backstage felt VERY uncomfortable about - but as long as they are doing thier job, and they are both consenting adults, then leave it alone.