Silver lip injury

Looks like re-injury of its lower jaw to the point of its body making a calloused spot to prevent any future injury. I've had the same ailment on a number of arowana, mainly because they were wild caught and skittish from being taken from the wild at an advanced age. It's probably rubbing on the glass, or running into the aquariums's wall frequently. IMO, it will heal, although the process will take a while. It also may get a bit worse if it keeps up the same habit.

My aro (a Lei) keeps his calloused up from rubbing the front glass. He is in the living room, so is always watching everyone looking for a sucker to feed him. If your guy stops rubbing it it will go away. Mine will start to disappear and then rubs it raw again. Look at it this way, if he is rubbing it on the glass to watch you, that means he isn't a nervous fish and might not be as likely to hurt himself. I keep telling myself that.