You should look into a frozen cube meant for tangs and other omnivores. I really like Emerald Entree. It has meaty foods as well as greens.
A lot depends on the size of your tangs, the size of your tank, and how much algae you have. If you have an effective clean up crew and proper flow, you shouldn't have enough algae for them and you should have to supplement their diets with algae sheets and/or greens.
I feed my tangs an omnivore frozen cube (Emerald Entree) as well as one or two cubes of a meaty food such as PE Mysis, Spirulina Brine, regular mysis, salmon, Marine Cuisine, as well as two 3" x 3" algae sheets every day. Now, I also have a large, heavily stocked tank and an excellent clean up crew, but still.
If your tangs are growing, IMHO, it is very important that they be fed the proper diet and be offered algae at all times, whether it is from the rocks or sheets. Optimally, you should have both if it's a juvenile tang. Most young tangs don't understand the concept of the algae sheet and it takes them awhile to get used to it and some will reject the clips if they're not introduced to it younger. A young tang may be able to survive on just the algae in your tank, but sooner or later, you'll have to supplement and it's a real pain to teach a stubborn adult tang how to use an algae clip.