Starfish; unsure of type.
Found in a big shell my son picked up, near the shore... we soon realised this little guy was hiding in it, so after these photos, he was gently put back with the shell where he was found it... as soon as the shell was submerged into the water the little guy scooted right back inside again.
Note: I added finger in shot, to show size only (not touch).
Madfish Bay in William Bay National Park