Very, very hard angel to keep in captivity. I had one for about three years, and then almost overnight it went on a hunger strike and lost all of its color. When I originally got mine, it had been in a dealer tank for over 3 months before I took it home, so it had already gotten past the first few weeks in captivity, in which the majority of Potter's succumb to the stress of collection (or at least that is what is theorized). Even so, it took mine several weeks before it started eating regularly, but once it did, it ate anything I put in the tank, including nori for my tangs.
If you've only had it a week and its picking at the rocks, I wouldn't worry just yet. But I would also be trying different types of food as well. Several companies make a blend of frozen food specifically for butterfly's and angels - have you tried that?