It could be a graze, hole in the head disease (hexamita) or fungus.
Fish fungus is normally white and fluffy and sticks up form the body. It gets into sores/ wounds. It doesn't look fluffy and if the fish has not been injured it is unlikely to be fungus.
Goldfish sometimes get hole in the head disease, which normally occurs in dirty environments. If you do regular water changes and gravel cleaning, and the filter is regularly cleaned, it is unlikely to be this.
If the fish has swum into something or scratched its head, it could be a sore. Under good conditions it should heal up without any problems.
The best thing to do is a 75% water change and gravel clean the substrate every day for a week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. Wash filter materials in a bucket of tank water.
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You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres 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.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
When you do the daily water changes, you will need to add salt to the new water so the level in the tank remains constant. eg: you fill a 10 litre bucket with water, add dechlorinator and mix it for a couple of minutes, then add some salt to the bucket, let it dissolve and then pour it in the tank.
If you don't want to use salt now, just do the daily water changes and gravel cleans and see if that helps.