As others have stated, Coral Beauty Angels are somewhat difficult; they are omnivores that require a special diet. As also pointed out, scrubbing your rocks and deco can cause a rapid imbalance with nutrients. Personally, whenever I've had a nuisance algae outbreak, I work on lowering the nutrient import and improve export until it dies off naturally. While GHA may look bad, it's actually keeping the water clean for your pets until you find and fix the problem. Your Angel was probably also grazing on it as they actually require more algae than meaty foods.