The ORA mandarins all eat frozen, BUT mandarins still need copepods in their diet. The fact that they eat frozen is just an added bonus. If you have a certified ORA mandarin that was supposedly one of the ones that they bred to eat frozen and it's not, apparently they'll trade it out for another one if you can prove it.
It's becoming more and more commonplace to find mandarins that are accepting frozen, especially if they're captive bred. ORA are not the only ones that have been successful with this, and they're not the first, they're just getting most of the credit. There is a breeder in the tri-state area that has mandarins that accept frozen as babies, and there is now two people on Long Island that have babies accepting frozen.
Again, even if your mandarin eats frozen, their natural diet is copepods, and they still need to be available for the fish.
A nano, is really no place for a mandarin. You should probably return this fish and shop at a different store, OR come on here and ask questions, because your LFS is feeding you a bunch of BS. I've had three mandarins die on me through the years, one was eaten my a condy anemone, one was killed by his "mate" (I attempted to pair a male and female, and they didn't like each other) and one was killed by a yellow watchmen goby. None of the three ever poisoned my tank, this is nonsense. Do they have a protective slime coat that helps fight off predators? Yes. Will it kill everyone in your tank? No, your LFS giving you bad advice will do that.