It's not clear from the photo, but if it looks like someone has sprinkled salt over the fish, that's ich.
The best treatment is heat; all the medications are harsh on the fish. Clean the tank to remove as many stage #2 parasites as possible, then refill with water warmer than usual. Turn the heater up to get the water temperature to 86 deg F/30 deg C and leave it it that level for 2 weeks or 1 week after the last spot disappears, whichever is longer. Then turn the heater back down. When you do water changes, use warmer water to keep the temp at that level.