My mandarin was sold to me as eating frozen/prepared foods. I have never seen it touch any prepared foods I add to the tank, even target feeding doesn't work. My mandarin is fatter than any mandy I have seen even eating frozen/prepared food. I however feed my tank plankton all the time so there is plenty of bugs for her to eat. Keep in mind just because it may be eating frozen in the pet store or you are told it is does not mean that it will eat frozen when you get it home. Chances are good but not 100%. Be prepared that you might have to supplement your tank with copepods and smaller systems (less than 100 gallons IMO) will need continuous additions of copepods if the fish is already in your system, they will wipe out a pod population like no ones business in small tanks.
Also, from my observations mandarins have a really low metabolism. It can take months to starve out a mandarin I think. I would consider a mandarin a "expert only" fish more so than a lot of other fish sold as expert in the hobby. They have really tiny mouths, are very slow to grab food and most require a continuous supply of live food to survive long term. Their survival rate is abysmal in home aquariums and should not be purchased unless you are willing to supplement their diet or provide a big enough system for copepods to multiply without preditation. I would consider my mandarin the most difficult fish in my system to keep (above the Moorish Idol) and she is one of the main reasons I started to culture live foods.