Hi and welcome to the forum
It looks like a bacterial infection (Columnaris/ neon disease), however that normally kills the fish within a day or two of them showing the white patch. These diseases also normally attack the fish and start killing the fish within a few days of you getting them, not two weeks later. You might have a slow growing form of Columnaris that came in with something else. This disease usually needs antibiotics.
The other possibility is a microsporidian infection, which causes the muscle tissue to turn cream/ white in colour over a period of time (weeks to months). This can be treated with salt.
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How long has the tank been set up for?
Have you added anything to the tank since you got the neons?
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You can either isolate any fish showing symptoms and treat them with something that kills bacteria. The remaining fish are monitored and if more get sick, you treat the main tank. Or just treat the main tank but that could wipe out the beneficial filter bacteria.
You can try adding salt to the main tank. It will treat microsporidian infections and can help slow or stop Columnaris from spreading, but it's not a 100% guarantee it won't spread.
If you do add salt, just leave all the fish in the tank and treat it with salt. Monitor the fish and if anything changes in the next few days, let us know immediately.
If it spreads to other fish and they start dying, you will need to treat the whole with an antibiotic.
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SALT
You can add rock salt (often sold as aquarium 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.
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, Bettas & 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, 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.
