If the fish is already in the tank, I think the safest bet is to pull every fish out of the tank, and treat them all for ich in a quarantine tank.
If you just pull the one fish out, and treat it in its own quarantine tank, you might get lucky. Chances are, though, that the fish had ich parasites that attached at various times, and some of them infested your tank in the last 24 hours. Read up on the life cycle of the saltwater ich parasite, and you'll see that it has several distinct stages. The parasites live attached to a fish, free-swimming in the water, and attached to the substrate at different times in their lives. If any parasites are now growing on your gravel, the other fish will be infested with ich all too soon.
Edit: Not everything that looks suspicious is actually ich. It could be that this fish is clean, and another fish in your tank attacked it. Posting a picture will help us guess whether it's ich.
The snails, corals, etc. will be fine, as long as you don't add any anti-ich medication to the main tank. One of the best ich-fighting ingredients is copper, which is deadly to many invertebrates. If there are ich parasites in your main tank, the best strategy is to wait to put the fish back in until the parasites have probably all died for lack of hosts.
A few fish, like mandarins, tend to resist ich very well, and can stay in the tank. Most mandarins also require live pods to survive, which makes life in a quarantine tank very hard on them.