Just a word of caution, be very careful to match pH and temp when doing a Freshwater Dip. RO water can many times have a lower pH due to many of the impurities that buffer our tap water being filtered out Just plunking the fish down in some RO water could definitely do more harm than good.
That being said, I am a big proponent of the FW dip method, especially when getting a new fish. If the fish is remotely healthy, it should be able to spend at least 10 minutes in FW. I have dipped a moorish idol for as much as 30 minutes. In my experience, I generally see the flukes start to appear in the first minute and they usually fall off after 3-4 minutes.
Regardless of all that information, a white ball is much more symptomatic of Lympho. Treatment for that is excellent water quality and excellent nutrition. My understanding of the disease is that it is a virus and not much can be done to combat it, outside of surgery in very extreme cases. At this point, I would not recommend a freshwater dip unless signs of flukes are present (heavy breathing, flashing, poor appetite,etc.)
I would do a water change and then be sure to offer some form of algae on a clip daily and also another quality herbivore food. I would add some Vitachem to the herbivore food, I like Ocean Nutrition formula 2 if you can find it.
Lastly, it is definitely an Atlantic Blue tang. Good Luck!