How did you aclimate it? There have been times when I was brand new to salt water when I did a lowsy job aclimating new fish and that shocked the them and then it wouldn't eat.
You may also want to try to spark its appetite with some garlic. Fish love it and it's good for their immune system. Kent sells garlic extract but it's outrageously expensive. I just use the juice from that diced garlic that you can get in the glass jars at the super market.