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.
my question would be whether it was eating at the store you got it from and then what was it eating there? Also, offering up a variety of foods from frozen to flake/pellet to different types of seaweed. I have 3 SW tanks and all get no less than 4 types of frozen foods mixed together with Selcon, green or purple or brown seaweed, and formula 2 flakes (I feed several small meals a day). Since doing this, I have never had any problems getting fish to eat on the first day including a subadult majestic angel I just got from the Diver's Den.
Unfortunately, as others have said, it's not sounding as though the PBT is doing well with not eating anything or even picking at the rocks.
Did the LFS guy feed the fish in front of you,and did it eat,sometimes they go on a hunger strike for a few days. Try brine shrimp,sometimes put fresh clam on a feeding stick and try to put in front of the fish.Dont give up. If the other fish in your tank are doing very well, then its probably not you. Also watch to see if its being bullied.Also these tangs need near-pristine water conditions.
sorry to hear your PBT never made it....ive had my PBT for a year now in my 100g system that he shares with an orange shoulder and yellow tang....mine eats anything i put in the tank but mainly is fed on sheets of brown,red and green seaweed sheets by ocean nutrition which he loves....he also goes crazy for flakes.....in my opinion it is a must when buying a tang from your LFS that you make sure its eating before you buy it....i was lucky that my LFS is very good and my PBT was in perfect condition and eating when i purchased him.