It could be a granuloma, caused by a bacterial of fungal infection, or it could be a tumor, hard to tell from a photo.
My veterinary textbook "A Self Assessment Color Review to Ornamental Fish" by Lewbart suggests to veterinary students, surgery if it is the later.
If it were me, I'd anesthetize the fish with a little clove oil, while continuously yet slowly pumping water thru the mouth over the gills, and simply lop the growth off with a alcohol sterilized razor blade, and swab the wound with iodine. Then place the fish into a small hospital tank until recovered.
But I'd only expect a 50/50 chance of survival at best, and that's being optimistic.