Fancy with growing white bulge on head

[MaggstAa]

My friend's fancy goldfish has recently developed a white lump on top of his body in front of his top fin that seems to be rapidly growing. His behavior has otherwise not changed. It has just appeared over the past week. I have never had this with any of my fish before so couldn't give him personal advice. Anyone have an idea what the problem is/if he needs treatment? Photos attached.

 

[SherLar]

Unfortunately, fish occasionally develop tumor, even cancer, just like people. It could be a simple fatty tumor that causes no danger to the fish, unless it grows too large, or it could be something worse. Either way, I doubt it's anything that could be contagious to other fish in the tank.
Some types of bacteria can cause lumps and tumors, also, I believe, though I've never seen it. My advice (someone else may have better) is to observe the fish regularly, and if the lump grows to the point where the fish seems uncomfortable or has trouble swimming correctly, you may need to euthanize. In the meantime, you could try a mild anti-bacterial remedy like Melafix if you wanted, and keep up on regular water changes.