I have a local friend who says it may be a tumour because it has grown, and it will need to be cut off. He also mentioned treating it with beta iodine, thinking it might be an infection.
That picture is a little more clear. It does not look infected but definitely looks like a tumor. If I had access to the fish I would still want to take samples to know exactly what i was dealing with.
I've had several GF and Koi over the yrs develop similar tumors. Some I was able to remove successfully others were a lost cause. Depends on location and size.
If that can be confirmed a tumor, its in a good place in terms of being able to remove it. Its not sitting right on top of vital organs and its rather small. If you have a someone experienced who can take a look and remove it I would go that route the video I attached explains the processes.