It either got its head caught under something and took the skin off, or heater burn is another possibility.
Are there any big fish int he tank that might try to eat it?
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Make sure there are no sharp objects in the tank and nowhere for the fish to get trapped under or behind.
Aquarium heaters don't have to be stuck to the glass with suction cups and mine were always put near the side but the bottom of the heater rested on the gravel and no suction cups were involved. This allowed the fish to swim around the heater and avoid coming in contact with it.
Do a big (75-80%) water change and gravel clean the substrate to dilute any disease organisms in the water.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Monitor the fish and if it gets more red around the area it is probably developing a bacterial infection. If it gets white fluffy stuff on the area it is fungus. Both can be treated with salt. You can either add salt after the big water change or monitor the fish and see how it goes over the next few days.
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SALT
You can add rock salt (often sold as aquarium salt), swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres (5 gallons) of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for 1 to 2 weeks.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.
