My harlequin rasbora stopped eating and has a red spot on his body behind his gill what is this?

This sounds like an abscess. This problem usually only occurs when a tanks is overcrowded, or the water has not been changed often enough, or a filter is not doing its job, or a combination of all of the above. There is no known cure for it. I would euthanise the fish and then I would make sure I am doing everything correctly. I'd allow a minimum of 1 gallon of water per 1 inch of fish and preferably double that amount of water. I would install a really good filter (the best I can afford) and I would change at least 50% of the water in the tank each and every week without fail.