The string of Caulerpa on the Algae Clip reminds me of another thought about the tank. The only reason I can think of that the Tangs would only be eating the "grapes" off of the Caulerpa Racemosa would be because they are just eating the tastiest or easiest part, kinda like someone that doesn't eat the bread crust. Again, it looks like you are feeding the fish very well(too well). Don't give them anything more to eat, except as a preventive against ich (especially on the new Regal Blue Tang), a little frozen food every other day with 2 Garlic drops mixed in. They will find other food right there in the tank, hopefully nusiance algaes and not too much sargassum/kelp.
P.S. I'll bet they will devour that gorgeous brown plating algae we looked at, though I can't for the life of me see why you would want that gone.
Edited by Mark Peterson