Saltwater Puffer fish are, regarding illnesses, pretty hardy animals. Nevertheless, it can happen, that new fish, that have just been inserted into the tank, which have possibly been sitting in the petstore for weeks or even months, develop white spots through stress. To avoid this, it has been proven, to feed them a variety of foods, and food, that has been treated with vitamins and garlic(-juice). Even if this illness has already broken out, in most cases, it can still be stopped.
As the most saltwater puffer fish have a high tolerance towards a low salt level compared to other saltwater fish, in a severe case of the white dot illness, you can treat it by lowering the salt level of the water. As this would possibly not be too good for other fish in the tank, it is best, to isolate the sick fish to a separate tank.