Is this fish in QT ?
Personally I keep some cured mature liverock in my QT for new fish to hide in and pick at, especially tangs.
Some people treat all their fish w/ copper and other chems regardless, but I feel like why add more stress if you don't have to. I watch mine for 75 days and only treat if I see symptoms, so could remove the live rock and let it dry and reuse if ich or something else develops and I need to treat. My yellow tang hid for the whole QT period and I never saw him eat once. Once in DT he was much more comfortable coming out and eating since other fish were also doing the same. That little bit of algae growing on the live rock though may have been what sustained him while in QT.