Pseudoceros dimidiatus - advertising its poisonous flesh to potential predators with its bright stripes. 

Close up of the tentacles of Entacmaea quadricolour - an anemone that is often host to several species of Anemone Fish. 

One of the many worms that burrow into coral for its home... feeding on suspended matter on the water with its fine blue gills that act like a net. 

One of the many animals that buries itself into coral by secreting a mild acid that allows the coral to grow around it but not over it, forming a secure and safe niche that makes it difficult for marauding large jawed fish - such as Buffalo Fish or Triggers, to extract them. 

These little creatures are indeed an animal.  They are a sea squirt and so are filter feeders - simple examples have one hole where water comes in, a basket inside that filters food, then waste water is pumped out through a second hole.   Compound ascidians are many "individuals" joined together basically, with many incurrent holes (the little dots) and one large excurrent siphon (the green hole in the middle).  This example Didenum molle, is one of the most common found on our Great Barrier Reef.  This one was at Opal Reef off Port Douglas. 

All the Echinoderms (meaning spiny skin) have a pattern of radial symmetry - usually with 5 points.  Starfish, sea urchins, sea cucumbers, sand dollars and feather stars are examples.  Starfish usually feed by crawling over the bottom using the hydrostatically operated tube feet on the underside of their arms, and everting their stomachs over their prey. Most are carnivores or feed on detritus. This one is Nardoa novaecaledoniae. 

A perfect blue star - Linckia laevigata - unfortunately one of the most frequently harvested in the Indo Pacific as a souvenir.  These starfish are usually found on the tops of reefs where they feed primarily on algae and detritus on the reef floor. 

Another relative of the starfish, Feather Stars are living relatives of the ancient Sea Lilies that were one of the first forms of life in ancient seas.  They are suspension feeders and filter water in their arms, moving it with tiny "podia" on a mucous trail that leads down to the central mouth.