As I understand it, the Ich parasite exists in most saltwater tanks, but tends to flare up from time to time, often in times of stress. Many people suggest supportive treatment (garlic extract to help encourage eating, careful maintenance of water quality), and the fish will often recover on their own, particularly if you can keep them eating. Now that the parasite is in the water chances are high that she'll get reinfected if you take her out and treat her.
Others have more experience with this than I do, but other things that I've heard are helpful: UV sterilizer in the tank to kill the free-floating life stage of the parasite, freshwater dip for the fish (very stressful for the fish and very hard to watch, not recommended).