possibly "New Holland Sea Fan? (Peyssonnelia novaehollandiae)... so red algae would be the broad classification. (I'm referring to book " Australian Marine Life" second edition by G. J. Edgar page 60. I would be keen to hear any other suggestions!
I think this is algae (although a bryozoan is not completely out of the question). Dive in main lagoon.Image is @ depth of only 3-5 m.
Some type of sea slug? Not sure. In all sizes (from 3cm across to 15+ cm) and some in clumps