Sometimes the fish get sand stuck to them and it can look like ick. Is the spot still there? Some of my dummies, especially my Formosa whos always digging in the sand, (even the clowns) will make you panic for a second thinking they have ick when its really just sand. Any chance it could be sand like my guy below?
A lot of people garlic soak the fish food before they feed it. You could try that.
