Hmm, 2 weeks is a bit long. Do you see him picking at the rocks? Leave the nori on the clip or try some romaine lettuce, they eat that too. I have found something for pick eaters and my mimic tang loves it. It's a caviar max by dr. G's. My Lfs
has it in the fridge by the pods
. Worth a shot. Your tank is definitely big enough and you said your water is good. Tangs can be very picky sometimes. They are herbivores, so sometimes they won't eat shrimp and other foods aside from algaes. Mine does eat mysis and pretty much everything. Keep trying, but don't add enough food to fowl the water. If you just add a bit the inverts most likely will catch it.
I did hear that garlic can entice to eat, but I have no experience with it. Maybe someone else can chime in??