Hi,
while feeding terrestrial vegetables to tangs and what not does not appear to do them any harm it does not really do them that much good either, as many of the vitamind essential to their health are not found in terrestrial vegetables, or even freshwater plants and algae for that matter. The same way as freshwater animals such as feeder goldfish or mysis shrimp are not as good for carnivorous saltwater fish as saltwater food. So why bother with lettuce or whatever when yu could just feed them something much more nutritious, such as nori or Spirulina flakes. OSI brand are good. You can also use them as phytoplankton to feed your corals.