Never seen a Powder Blue really like Nori. Nor do they like flake. They are more of a meat eater than a vegi eater. This week I have 6 powder blues and 2 powder browns at the store I work at. Since I am the designated feeder I watch closely on what fish eat what food and with the powder blues they only seem to eat off the rocks, or they eat original Rod's Food which is mostly meat. Seems odd for a tang, but that is what I see on a daily basis. Powder Browns are a little different, they do seem to eat nori readily.
As already stated, Powder Blues are VERY hard fish to get established. I usually tell people to expect to go through 4-5 of them before they get one that lives, and that is even after they are eating good at the store and look healthy. The transition to a new tank is very stressful on them and that is the point they seem to go down hill. That is one fish that I feel needs a 240G+ tank to be truely happy. At the store we have one in our 400G reef with 3 other tangs, yellow, kole, and purple, and they are all very happy in that size tank, but we tried several powder blues in our 175 bowfront and none would make it, just too small of a tank for a powder blue. The Powder Blue is the owner of the stores all time favorite fish so that is the one fish we must pay special attention to as far as feeding and health.