Is it sticking up form the body?
Can you post a couple more pics from different angles?
It kind of looks like an anchorworm and a big red patch. The red part could be the start of an ulcer (goldfish ulcer disease), or it could be from an anchorworm or fish louse chewing on the fish.
Have you added anything new to the tank in the last few weeks (plants, fish, shrimp, etc)?
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If it's Goldfish Ulcer Disease (GUD) you need a medicated food to treat it.
If it's anchorworm or fish lice, you can kill it with salt. Salt should help with minor bacterial infections too (GUD is a bacterial infection).
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 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.
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Before you add salt, wipe the inside of the glass down and do a 75% water change and gravel clean the substrate. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash filter media/ materials in a bucket of tank water.
After adding salt, you should do a 75% water change and gravel clean the substrate every day for a couple of weeks. The new water should have salt added to it before it's poured in the tank.
The big daily water changes and gravel cleaning the substrate will help reduce the number of disease organisms in the water, and should give the fish the best chance of healing itself. If the red patch gets worse, then you will need to get some medicated food for the fish. However, I am hoping the salt and big water changes should fix the problem.
