Not to play devils advocate, and I hope your fish isnt sick, but doesnt ich disappear like that anyway? I mean, it shows up on the fish as spots for only a day or two, which is the only visible stage of its lifecycle, then falls off and disappears as it reproduces......only to re-appear again on fish a few weeks later?
I have never had marine ich, so I am definitely not an expert, but my boss is dealing with it in his tank at work, and I have seen it disappear completely from the fish, only to reappear several days later, or the fish dies suddenly. Then other fish start getting spots and it starts all over.
Hopefully I am just overparanoid and your fish is fine.