Patience. This family of angelfish can take some time until they eat. My 8" blueline took I think almost 7 weeks until I saw it eating and now it eats anythin meaty like crazy. Green foods like veggie flakes, spirulina or Nori sheets is a good try. One of my favorite foods is Hikari mysis (try the larger kind also) and as stated above clams.
I highly recommend a FW dip to check for flukes, as this can lead to a loss of apetite and by the time you figure out is has flukes it is very often too late. It is very likely that angels have flukes. If they don't have it from the ocean they often get it from the various wholesalers and LFS, as hardly anyone treats for them.