The Nereids are minor goddesses of the sea. They get really ticked off when Cassiopeia brags that her daughter Andromeda is more beautiful than they are. When they complain to Poseidon, the great god of the sea threatens to destroy Ethiopia with a flood and a giant sea monster.
The Nereids aren't the only goddesses to fly into a tizzy over a mortal girl's beauty. Venus, the goddess of love and beauty, did the same thing. Read about Venus' jealousy of Psyche here.