tangs will nibble on just abotu anythign that is algae, except for coralline. Ours go just nuts for caulerpa. I have tried to get a strand growing in my main tank for years, but as soon as i put a cutting in there, it is devoured! They also love red algae, and will from time to time nibble on sargassum, which is a brown algae relative of kelp, but leave coraline and cyan failrly well alone!
good luck
jon