If it looks like a small (1/8" or so) cloudy bump on the fishes eye this is usally a parasite. Depending on the type of parasite a freshwater dip usually kills them but can be stressful on the fish. If you have a QT there are some medications that may be able to get rid of it as well. Also, if it is a parasite there is probably many more then just the one you have noticed so keep an eye out on all your fish.