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For new fish, getting them enough calories is the key, you can work on diversifying its diet later on. Don't leave the mussels in the tank so long that they decompose - perhaps a couple of hours each. You can also try pressing other food items into empty mussel shells to try and trick it.
Looks like it has a bit of Lymphocystis on its right side and right pectoral fin - tough to see though. That could get worse, but it is self-limiting and will go away on its own without treatment.
My big worry for this fish is that emperors that haven't been quarantined very often end up with ich (Cryptocaryon) or Neobenedenia flukes.
Jay