Coin collectors will always tell you never to clean coins, but it's your decision. Just be aware that if you end up selling your coin to a collector, you should disclose that you cleaned it and that having done so might dissuade some of them from buying it.
As far as the cleaning goes, I've cleaned some coins before and sometimes soap and water does the trick. If not, some brass polish will definitely work. You can get some of that at your local hardware store or maybe a Walmart or similar big-box store. But I probably wouldn't do anything. Do you have a picture of it by any chance?
Are the brass coins (and silver coins for that matter) prone to tarnish in general, if not kept in an airtight storage container?
I don't have any brass coins, but I have a brass pharmaceutical weight (1kg) that I cannot keep from tarnishing. I've polished it, but even then I can't remove all of the spots and I've stopped trying. I've always thought brass was more resistant to oxidation than silver, but apparently not in my house.