Welcome to the boards Bobby.
The Rock Beauty is indeed one of the hardest angelfish to keep alive due to the fact that they eat tunicates, sponges, zoo's, and algae. In our tanks we can usually supply them with algae, but the rest can be very expensive to feed and without proper lighting will wither and die before they can eat them.
But, with this said, it can be done. They make a frozen food for angelfish that is mostly a sponge diet. This will help tremendously. Supplement the food with vitamins. Of course nori is always a good staple to give them. This can be bought in sheets and cut to size for the tank, and or fish. Mysis is always a good food to add, but not all will accept it, these tend to only want a sponge diet.
You have a beautiful fish and with care will be a good investment, not to mention a center piece for the tank. But, probably the most important factor in their long term survival is a prestine tank as far as water quality. They need extremely high quality water, and good flow. Good luck, and keep us posted.