Ok, first the facts. Whether a mandarin is eating frozen prepared foods is irrelevant. Totally. It cannot get enough nutrition this way absent sufficient copepods. Secondly. A thirty gallon tank will be fine with a refugium that produces sufficent copepods. In fact that environment is better than a larger one with competitive eaters (of copepods)
Now as to the white spot. It looks as if he might have been stung and his slime coat may have protected him somewhat. As long as he is eating, there is hope. It is not really feasible to isolate him for treatment because he would starve to death in a tank deficient of copepods.
I am a firm believer in ethical treatment of mandarins but before criticizing, it is best to have your facts straight.