Can I describe the card when using ability of Felline Cat?

Yes — you can describe the spell instead of saying its exact name, as long as your description uniquely identifies it. Per Rule 761, naming a card works two ways: stating the exact name, or identifying a combination of traits that pin down that card (e.g. "the 6-Might Kai'Sa" or "the 3-energy spell that draws two"). For Fallen Feline, that means something like "the 2-cost Calm spell that kills a unit" is legal if only one spell fits. If two spells could match your description, the name is ambiguous and doesn't lock in a specific card — so keep the description specific.