Whitespot is a PITA. The best course of action will be to quarantine that fish in a hospital tank ASAP. If you don't have a spare tank, I'd recommend getting one. It only has to be a cheap second hand thing, that will allow you to observe the fish.
Fill it with water from the main tank. This will help reduce the amount of parasites from the main tank and secondly, it will mean that the stress on the fish will be significantly reduced transferring it from one tank to another. Place the effected fish into the new hospital tank. Add a marine whitespot treatment at the correct dosage and follow the instructions for repeat dosing.
If you can, raise the temperature as high as you can in the main tank without affecting any of the inhabitants. 28c should be fine. This will speed up the life cycle of the parasite and hopefully, if it doesn't find another host, will die off. Keep observing the fish in quarantine and more importantly, those fishes that remain in the display tank. After about 2 weeks of the fish in quarantine being clear and no fish in the main tank contracting it, the fish should fine to return to the display tank.
If other fish have ich, it might be better to just quarantine all fish and treat for whitespot in the hospital tank, leaving the display tank void of fish for 2 weeks. The parasite is only treatable when in its reinfection stage IE when its waterborn and seeking a new host.
You could attempt hyposalinity treatment, but unless you have performed this before or have an experienced fishkeeper to give you help and advice, I'd stick with the medications.
In the future, if you purchase new fish, put them in a quarantine tank for a couple of weeks and treat as if it is sick. Then add to your main tank. Nothing more gutting than having hundreds of pounds worth of livestock wiped out by one infected fish.
DONT be tempted to add copper based medication to your display tank if you have live rock, invertebrates or corals.. as you'll most likely kill them all.