I will update this post as time goes on, in case it might help anyone else in the future.
Two photos below show Cannonball's progress. One is in mid April when it was looking so much better. The lump seemed to shrink, not fall off, but the edges of the area stayed a bit raised.
The other photo is today, 26 days later. As you can see from the photo, the lump has returned, larger than before. It is also darker than it was. He also has a small white lump on his head. In the photo, it just shows up as a blur on his face, but it is a small lump like the other. It obviously has the potential to become really bad, especially considering the location.
I am keeping an eye on him and his quality of life (as best as I can understand a fishes' quality of life). He still swims around inspecting everything and eats his food and the snails that are small enough. Although, I have noticed the past couple of days that he hides a little more than usual.