A freshwater dip will force some of the cysts to drop off the fish, but that's not a cure. You must get the fish into a quarantine tank along with any other fish in the tank, and CURE them with either copper or hyposalinity treatments. Those are the only 2 scientifically proven methods to kill cryptocaryon irritans (saltwater ich). You must also leave the main tank fishless for 6-8 weeks. If the parasite doesn't have a host fish, it will die in the 6-8 weeks with no fish in the main tank.
Ich is such a simple problem to avoid. All it takes is not putting a new fish directly into your main tank. Take 2 weeks to watch it in a qt tank and treat if necessary.