dime...is this a new fish that was just recently introduced into your tank? And, is it a reef or a fish only tank?
All fish should be moved to a hospital tank and treated. The main tank need to remain fish free for 6 to 8 weeks tp make sure the ich has gone through its life cycle and died off without a host to live off of. Check the ich sticky atop the forum for some great info. Good luck, always go slow and make slight changes as not to stress an already stressed fish more and always quarantine any new fish.