It's very likely that this fish will starve if you don't have a good source of live copepods. Most people have a refugium just so they can breed pods
but it's no guarantee that you will be able to produce enough pods
to sustain a mandarin. You can possibly feed it live baby brine shrimp but that's a large pain. Even if you do set up a refugium it will take quite a while to seed it with a decent pod population. You could also take a look at how to train them to eat frozen food.
I would honestly suggest taking it back to the store and buying a different fish and in the future ask to see them feed a fish such as that before going out and buying one.