The problem with feeding them seaweed is that my purple tang will rip it off the clip. It breaks up too easily and most ends up drifting into the filter. If you know of any seaweed that is "stronger", let me know...
I agree with the not using lettuce and switching to nori, but to answer your original question, a heniochus will pick at it definitely, but a tusk most likely will not, as he is interested in more meaty foods.