Port MacDonnell. Shallow limestone reefs and substrate which lowers wave energy. Small rocky bay surrounded by reef. In rock pool and among rocks at medium tide on inter-tidal limestone reef. Rocky headlands with small sand pockets.
Using the suggestion for this ID. Looks like the inside of a smashed clam. There were many sea turtles feeding nearby if that helps.
education.busseltonjetty.com.au/fish/dusky-spotted-flatworm/ calls this Eurylepta fuscopunctatus "Dusky Spotted"
www.gbif.org/species/4859796 says that's a synonym, but note lateral stripes at margin, contra dots in M. f.
South end of Island Georges Bay