My betta injured itself

Hello, my betta recently just recovered from fin rot and has been doing well. However, today after I went out for a bit when I came back he injured himself. Nothing is sharp in his 10 gallons tank and no other fish either. On the side of his body, it looks like he rob himself against something and as a result, he damaged his scale. His side body looks like someone scratched your car. I wonder if anyone know why this is happen? I wonder if it is because he sees too much reflection of himself.

 

First things first- his fins look great considering the recent recovery from fin rot. Now for the mark on his body: I can't quite tell if its just a few missing scales (which could have just been an accident, maybe something spooked him and he knocked into something while darting away) or if maybe its some kind of fungus. Could you get any closer photos?
Assuming it is just missing scales, I would just make sure you keep up with maintenance, keep water conditions good, and they should heal rather quickly.

 

Can you fill out the emergency template to give us more information please? Also include everything in his tank

 

it’s so hard to the a picture of him because he keep swimming so fast. I observed him for a bit and I noticed that he would swim away from the glass and then swim really fast toward it. I’m thinking this is how he injured himself? I used to have a black background for his tank and just removed away 2 days ago so maybe he’s not used to it?

 

With regards to why it happened, bettas are curious fish and will often try to squeeze into small spaces or he could've bumped into something. I would double check your tank for sharp rocks, wood, decor or plastic plants. Does the patch on his body look fuzzy at all? I agree the coloration isn't typical for just a missing scale, the flesh underneath is usually pink from my experience.