Aww, sorry. He even looks sad. I'm not very good at diseases, but I'd guess some type of bacterial or fungal infection. API makes a cure for broad range bacterial-fungal infections. Go immediately to a fish store, show they the pics and get some meds started. Try not to get your help from a PetCo/PetSmart, unless you know they have a knowledgeable fish person working. You should just treat your entire tank, because the other Angels are likely infected too. Most disease are caused by poor water conditions. Keep up on weekly water changes of 40-50% to keep something like this from returning. Let me know how they are doing. Good luck!
I posted some pictures of the sick fish but I'm not sure you can find them they are in my album it took awhile for me to figure out how to get pictures onto this web site, was not easy. Hopefully you can see my sick fish.