Believe it or not, this actually is an animal not an unusual pillow.
It is an unusual sea star known as the Square Biscuit Sea Star. They are
found in coastal waters around Australia up to a depth of 40 meters.

 Square Bisuit Sea Star (Photo via Unusual Facts)Square Bisuit Sea Star (Photo via Unusual Facts)

are closely related to the Biscuit Sea Star which has one additional
arm. They feed on sea squirts, sponges, bryzoans and algae.

Photo via Unusual Facts. Source: Australia Museum