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That's a blue velvet nudibrach. It only eats flatworms. So you'll need to make sure that you have flatworms in your tank, which isn't generally something that reefers like in their tanks. Personally, I would take them back to the store.
Probably for the best. I suggest you Research what you buy next; it's a hassle buying things and then having to catch them to take them back after finding it won't work in your tank.