It does look somewhat like it could be. First thing to do, soak her for 10 minutes in a betadine water solution [ mix to look like iced tea ] and use a toothbrush to see if it comes off so to be sure it's not just a stain. Then pat her dry + put her in her warm tank. If the dragons live together, separate them ASAP. If this is Y.F., don't
bathe her anymore unless she is obviously dirty, walked through poo or something, and use betadine each time.
An experienced reptile vet can do a skin scraping to be tested for Y.F. Be sure the vet really knows about beardies, otherwise you will waste your money which some vets will happily take while giving wrong advice. If the spot is stil there after you try to clean it, you can use Monistat or Lotrimin antifungal cream on it for now. Best wishes !