Is Medusa a Greek goddess

No, Medusa is not a goddess.

She is a mythical creature (monster). She was the daughter of Phorcys and Ceto.

According to Greek mythology, Medusa was originally a beautiful maiden, and a priestess in Athena's temple. She was described as "the jealous aspiration of many suitors" by Roman poet, Ovid. But when Athena caught her laying with Poseidon, the Lord of the Sea in her temple, she, in her rage, punished her by transforming Medusa's beautiful hair to serpents and made her face so terrible to look at that the mere sight of it would turn onlookers to stone. Along with her, Athena also, cursed her two sisters. Anyone who looked at them would turn to stone.