This is Poseidon's weapon of power. The trident is generally used by fisherman to spear individual fish, but the Trident symbolizes Poseidon's power. With the Trident equipped, Poseidon gains his power's.
As for Poseidon's appearance, he is often seen wearing a crown. This could or could not affect his powers. If not, it could be simply used for decoration.
Poseidon has pure white hair, meaning he is very old in age. He is also seen as being physically fit, suiting the tile of a god. He also has the tail of a mermaid, which can be used for a more efficient way to swim.