How is she today? Where you able to make any headway on this mass? If you are able to clean it up, and you get down close to the skin, make sure to dig out any ingrown feathers, if this is the cause of this infection. Ingrown feathers are quite common and do cause infection. Most likely any...
Use sterile instruments to dig around in this, make sure your hands are clean or use latex gloves. If you have any triple antibiotic cream (without pain killers) you can apply this to any open infected wounds.
Twin Cities Poultry carries a few different types of antibiotics for poultry. Here is a link to their powdered Amoxicillin put in the drinking water. (They also carry capsules as well)
https://www.twincitypoultrysupplies.com/index.php?main_page=product_info&products_id=1196
If it crusts off, be prepared with rubbing alcohol, iodine or some sort of saline solution. If you can it needs to be cleaned out down to this skin. If this is infected you might start the bird on Amoxicillin or Penicillin too.
But if this thing bleeds, you need to just leave it alone, tumors...
I would towel her up tight with this exposed and pick at it a bit. If it bleeds, its probably a tumor of some sort. If it peels and picks off, it may be dried pus from an old injury or ingrown feather.