Had this issue with the white antenna bands on the HTC One M7. Could have been from a pocket or a towel or anything.
I used a small amount of rubbing alcohol on a cotton ball and (lightly) wiped them clean every month or two. However, I'd be very careful the first time you do it. Don't want to scratch it or leave any markings on it. I also know some guys used a Magic Eraser on their M7's as well. Though, they do have a light abrasive in them that you should be careful about.