Poseidon is the brother of Zeus, and he's the great god of the sea. When Cassiopeia offends the Nereids, he threatens to send a flood and a sea monster to destroy the kingdom of Ethiopia.
Is Poseidon being especially grumpy in this story? Well, not really. He's known for throwing temper tantrums, especially when mortals offend him. Don't believe us? Just ask Odysseus.
For much more on Poseidon and his anger-management problem, click here.