Our purple tang really doesn't like the clip. If I rip chunks off he will eat quite a bit. I feed LRS reef frenzy so he gets most of his nori in the frozen food. Sometimes if there's another fish that will eat from the clip then your tang may follow suit. Also, you can try to rubber band it to a rock and see if he nips at it.
What kind of tang? The bristletooth varieties have a harder time tearing at nori with their special teeth. And some tangs eat more plankton than they do algae.
I just trained a scopas to eat nori after a couple months of struggle. If there's another tang in the tank to teach them, they tend to catch on quickly, but my scopas is the only tang in my softie tank and it took him a minute. For a fish that is used to finding algae on the rocks, it can be confusing for them to think about going towards the surface to find food.
Tying the nori to a rock and sinking it can help to train them. I'd stick with it. Roll the nori up like a cigar so it doesn't disintegrate as quickly. The fish will figure it out eventually. In the meantime, maybe try Formula Two flakes and frozen. Good luck!
+1 for red sheet. For yellow tanks cutting up the Nori and soaking in a little Garlic and Selcon may help wean the tang on. Also I found placing the clip close to where the Tang hangs out works. I think they feel a little exposed if the clip is high up closer to the surface
Most of my tangs have eventually taken nori off the clip. Took my Chevie almost two years, others much faster. Not a natural way to feed so requires learned behavior. Feed a balanced diet and keep at it.