caveat: I am by no means an employment lawyer, careers guidance councillor, or anyone with any relevant qualifications! I merely offer this as my own personal opinion, as someone who has been employed in a variety of organisations.
I respectfully disagree with the commenter who says this "is absolutely not your problem". You are right to care about the wellbeing of your colleagues, that's one of the differences between a company (and dare I say it, a 'team') and a collection of freelancers. As far as you know, you've - inadvertently - uncovered evidence that your company discriminates according to gender, and that's a serious matter. Maybe it's not illegal, maybe the company has no real problem with it, but you do, so I feel you have every right to bring it up.
What might happen as a result, of course, depends on the company culture and the opinion of your supervisor (and their supervisor, etc.) The best case is that they've made a mistake, will be very grateful to you for bringing it to their attention, and your colleagues will thank you for your intervention. In the worst case, they'll fire you for meddling where you shouldn't and, if that's illegal in your jurisdiction, you'll need to decide whether to open a claim against them - either way, that case doesn't sound like fun.
Another option is to have a candid discussion about salary with one of your female colleagues. Your company might well discourage that kind of behaviour, but I'm pretty sure there's nothing illegal about it, and it's certainly not immoral IMO. If you're talking to the person you know is being paid less, they might appreciate you empowering them to deal with it however they wish, rather than fighting their battle on their behalf. Although be prepared to offer support if it's requested. After all, if it's really an insignificant amount, they might want to leave it, and you might be OK with that.
Ultimately, I think you need to tread carefully, but evaluate a) what sort of company you want to work for (if they react badly to you raising this, maybe they're not worth it, if you can afford such principles) b) what sort of colleague you want to be.