The unusual behavior is more likely an indicator that something is wrong vs a light colored beak. If you can catch her and transport her to a veterinarian with avian experience, that would be the only way to know for sure.

Another possibility is that this is not the same swan. Some bird flocks somehow understand when other birds are being fed and may show up at the food line. The bird that you normally see may not be in the area at the time that the new intruder shows up. This would prevent you from knowing if this is the exact same swan or a new swan. There is also the possibility that this new swan may be the mate to your swan. Your swan might be on the nest and the two mates will share nesting duties to allow each other time to rest, bathe and feed.

The Regal Swan