is it smooth and hard looking, or soft and irregular margins? is it red or sore looking? does it look like a cauliflower? for now, keep the water nice and clean. look at lymphocysitis. its viral, can be transmitted to similar species if it ruptures. no treatment. no treatment for cysts or tumors either im afraid. but may be something else altogether.
is he/she still eating and otherwise acting normal?
all the best