Naso's can be really tricky to get feeding, and it's not unusual for them to go on hunger strikes. My experience with fish with internal parasites is that they do eat, but don't gain weight - so I doubt that's the case here.
I would continue to offer nori as frequently as you can, and try as many frozen foods as you can. My naso has developed a taste for the larger PE Mysis. Have you tried that?