Actually I'd be curious to see how you got him to eat flakes too, kah!
I have a 4 in. derasa clam that my copperband has never shown any interest in. Copperbands are a common solution for eating Aiptasia (he cleaned out my tank of the pests in a week or two which is why I got him in the first place), however that means they do eat other reef things. Mine constantly is picking at the LR, though I can't see what he's eating (probably copepods, isopods, etc.). He has never shown interest in my rose anemone though...
I've heard rumors that they may eat tube worms, such as feather dusters and such (I'm experimenting with feather dusters and my copperband as we speak so I'll let you know).
He does seem to have a voracious appetite however and is constantly coming to the top of the tank when I come by hoping I have some food...my take on the reef safe idea is that any creature that is known to eat another reef creature (such as Aiptasia) may well find some other tasty snack in something your actually paid money for. I think it's a crap shoot...
Hope that helps...
Peace...
Mike in CO