You need to place the starfish in the aquarium in Poseidon's domain on Mythology Island. Look for the area with the aquarium and interact with it to put the starfish inside.
From the museum in Mythology island.
Use the starfish as an offering on the altar, by the door to Poseidon's realm.
Mythology island, on a statue in the museum
Go to the Gods museum on Main Street. You will find the starfish on one of the statues.
the pink starfish
It is in the museum on a statues head.
It is in the museum, on the face of Poseidon's statue.
The starfish on mythology island is on the face of the statue of Poseidon in the place right next to the balloon when you get on the island.
In the golden temple go in and you will find the starfish on one of the statue's heads.
You have to get the starfish and put it on the offering alter.
From Poseidon's statue in the museum.