Angels are grazing fish, and yours obviously prefers to follow his natural instincts and "graze" on your rock! Angels are also vegetarians, so you need to supply a nice supple of green vegetable food. I'd try 2 things:
First, get a lettuce clip and offer your angel rommaine lettuce of spinach. This food will satisfy 2 needs---grazing and green vegetables. The other thing that you can do is try to "hand feed" your angel. Only, instead of using your hand use a syringe. I have a clownfish that was being intimated by another male clown and refused to come out from his hidding place. Using a syringe, I was able to get the food right up to him, and he was able to eat [and survive, I might add]. This method has proven, for me, so much more efficient than throwing food in the tank, that I continue to use it. Basically you can feed small amts of food to fish, placing the food exactly where you want it to enter the water. My fish are so used to this now, that they see me coming with my syringe and surrouond it as it enters the water.