It could be getting ready to fall off. Is the thing on his eye transparent or solid white. Ich RARELY attack the eye-I actually don't even know if it is possible for them too. Never have more than 1 fish in a quarentine tank otherwise it is doing no good. Why is a Tang in a 5.5? That will stress them out WAY more than it will do them good. If you are having a problem with ich better ways to cure it (especially if the whole tank is breaking out) is to do hyposalinity (low-down near the 1.011-1.013 range-but do it extremely slowly) or to do freshwater dips (5 minutes tops in general-you could go longer in severe cases) or soak the fishes food in garlic. Any will work better than being crammed in a 5.5. JMHO. Anyway, hope this helps and good luck.
