I know I'm late but I'll try and help.
A tumor is a growth. Just so you know. Thats most likely a benign tumor, but you need to remove it since its getting so big.
Best case scenario, you have the resources for a vet experienced in fish to take a look. But thats unlikely.
Assuming that the stem doesn't branch deeper into the fish and is growing separate from the muscle, sever the growth at the surface. Use a clean scalpel or razor. Anesthetic is ideal for this (eg clove oil, benzocaine, etc). I don't know the dosage though, which needs to be precise or you'll kill him. He needs to fast for a a day or two before using anesthetics.
Good luck