What you're seeing is callus. Scar growth.
Your plant is trying to heal itself by creating lumps of cells that later become functional structures eventually but for now they're undifferentiated.
You can scratch it with a super sharp blade and try to keep it from protruding further but the cells below might not survive the exposure to the elements. It's better to wait for a bunch of months, or to just leave it as is.
It just happens sometimes when stuff gets damaged. If you check out air layering techniques, you'll see that a lot of people get that callus growth with the goal of it producing roots as functional structures, this is the intermediate phase.
The title had me worried about apple cedar rust, but that would be orange and spongy.