This is Nembrotha cristata, a colourful sea-slug found in the tropical Indo-West Pacific Ocean. They grow to about in length and have black bodies with a strange luminous green "trim". They deliver a painful sting.
The aptly named Ping-pong tree sponge (Chondrocladia lampadiglobus) is a carnivorous sponge. Those ping-pong ball looking things are covered in tiny spicules which the sponge uses to catch tiny crustaceans.