Ich is a parasite, if it is showing on one of the fish there is a good chance that its already in the tank if not already in the fish.
There are a few methods of taking care of ich, you can add a small amount of salt to the tank (google or someone else might know the amount, im not sure i believe it was 1 tsp per 5 gallons but please look it up im nowhere near 100% on that)
another is slowly increasing the temperature to above 86 F this and the salt method can be used together however i would recommend looking to make sure that that temperature is not to far out of what your fish can live in as i personally lost quite a few tetras by increasing the temperature
Alternatively you can use medications that you can buy from the store however do not increase the temperature and use medication as each treatment reduces the avaliable oxygen in the water and together you may suffocate your fish
I wish you the best of luck