My common pleco, named Fossil, spent his whole day willingly stuck on the wall, with his back (just above the dorsal fin) touching the heater. When I put my net in the tank to nudge him away from the heater, I noticed a white spot on his back. I looked closer, and saw a burn! I moved the heater from vertical position to horizontal, near the water surface. Most of it is just his scales burned away, but I can see some skin exposed.

What do I do?

(The burn is just above his dorsal fin, in the middle of his back.)

Update

In response to @Allerleirauh:

Could you add another picture, where the spot could be seen better?

Try it to hide behind the heater? (less possibilities to hide in the tank) or does it search for warmth? What do you assume?

I assume it was trying to hide behind the heater. There were not a lot of hiding spaces in the tank at that time. I only had a couple of plants in the aquarium, so I have added two caves since I saw the comment.

And what temperature is the heater deliver?

I do not know. It was set to heat the tank to 76-78 degrees Fahrenheit (24-26 °C).