That diet is highly insufficient, especially for a herbivorous fish such as an atlantic blue tang. There should be a ton more items in that fish's diet. The brine shrimp plus (Ocean Nutrition brand) is a good source of food, and the mysis shrimp is okay, but the regular brine shrimp should not even be considered one of its main items in its diet. I would strongly recommend you go to your LFS and pick up some freeze-dried seaweed, and at least 3 frozen food items. I feed my fish 5 different frozen formula foods because they contain the highest blend of different seafoods.
If your LFS is carrying brine shrimp plus, they are most likely carrying other Ocean Nutrition products. Pick up some Formula 1 or 2, Prime Reef, and VHP formula. These will give your fish a huge variety in their diet. They will be getting kelp, spirulina, spinach, algae, krill, squidd, clams, mussels, scallops, shrimp, and a lot of other goodies. Then, you will have the algae sheets (seaweed) for the fish to graze on. I would recommend trying Julian Sprung's Sea Veggies. His seaweed products are extremely high in vitamin C, as well as other important vitamins that your fish need.