White spot is an ailment that fish can display and the symptoms are a covering of tiney white spots over the covering of the body and gills. Regal Tangs are prone to getting this.
Here is an example of a Regal Tang suffering from white spot.
White spot is caused by a parasite called Cryptocaron irritans and this can be treated like many ailments in fish. This parasite is only vulnerable to medication during its free-swimming phase. Many medications are available that are reef safe and can be put directly into the aquarium if you don't have a quarantine tank.