Reef Aquarium Fishes by Scott Michael, page 300: "The juvenile A. fuscus is bright yellow over the head, body and fins, while adults are dark brown to black. It will attain a length of about 5.5 inches..." It has a picture of your cute little yellow fish and a picture of a larger black adult. Also says it is a grazer of filamentous algae, forams, detritus and sand, among other small things. Says to offer it frozen foods that contain spirulina, but if yours is constantly grazing on the rocks I wouldn't worry about it as long as his belly is not sunken.
HTH