CMV is refering to a recent incident where I had related to her about an injury involving one of my Black Stars.
One of my Black Star hens got pecked hard above the left eye and the area quickly swelled to the size of a marble. I suspected it was similar to water on the knee and needed draining. I wrapped the hen snuggly in a towel and soaked the eye with a wet warm washcloth to soften the swollen area around her eye. I used a syringe with a sterilized needle to go through the eye and into her eyelid in the upper left corner of eye successfully puncturing the swollen membrane releasing water from the wound. The swelling went down immediately and there was very little bleeding. I soaked her eye once more with the wet warm washcloth, then flushed her eye with homemade saline solution. I then put a small amount of neosporin in her eye for the next 5 days as well as dosing her orally with 1/4cc amoxicillin once a day for 5 days. She fully recovered without any issues.
I dont know if your bird was pecked in or near the eye or not, but you stated that you've observed it growing for over a week which leads me to believe that she was not pecked and possibly could be a tumor or something else. You could try lancing it if you wish and see what, if anything comes out. Then flush with saline solution and apply an antibiotic ointment if you wish. Good luck.