Usually I apply Japanese Yellowpowder on wounds or cuts after operation procedures on arowanas, not sure if its available in your country but overhere we use that a lot for all kinds of wounds for fishes
I had a similar bump on a flagtails head. It shriveled and fell off after awhile and left the fish no worse for wear. If you feel it's getting worse and want to do something you could always sedate him with clove oil and remove it. Good luck.