from the pictures it just looks like he ran into something. I didnt look at what is in your tank, but its possible he got in a fight with another fish eating or during the night. Or he might have burned himself on the heater. But to me thats what it looks like and as long as it doesnt affect the fish, i wouldnt be overly concerned. Id watch it to make sure it didnt get worse, and hopefully it isnt a permanent scar
Sorry to see it