LOL, we refer to them as Messy Mares, and just as dirty as they get, I bet their stalls are also the messiest. I don't know why some mares get so dirty but it just seems that they do. And they are the ones whose stalls we hate to clean the most. I'd just clean her good with soap and water once or twice a month and the rest of the time, just a wipe with a wet cloth on her udders. If you're talking about her back legs, it's probably because she's urinating on a hard surface and it's splashing back up on them and if she's in heat (some mares seem to never go out) she's also urinating down her back legs. They can get pretty awful. Just keep washing them, it won't hurt anything. And wash her tail real good too, that gets wet and yucky and hits her legs and private parts.
I'll take a nice neat stallion ANY day!