I have an update. The "tumor" hasn't gotten noticably better or worse, but I have done many water changes and the SG, nitrates, and pH are now corrected.
Today I spoke to the owner of Southern Illinois Tropical Fish Hatchery, someone I trust, and he thinks it's an ulcer. He also said that the pigment loss (apparently due to stress) was a sign of an ulcer. Hopefully continued water changes will help my goby get over his problem.