3 weeks is still a little soon to get frustrated. They can be shy fish at the beginning. I wouldn't worry about him, if he is eating Nori. Maybe try and go 5-6 days without the Nori and just feed the tank NLS pellets and Flakes and see how he responds. Also when you feed put the food in and move away from the tank, go hide around the corner or something, but still be able to see him eat.