Nudibranch

This is a juvenile Nudibranch, very small in size, around 1 cm.
The key part to see that it is a Nudibranch is the the 2 Rhinopores at the head of the Nudibranch. In this case, you can see 2 whitish 'horn' poking out at the front and the 2 oral tentacles near the base.
Nudibranch under the genus of Phyllodesmium are predatory sea slugs and often often show extraordinary mimicry, each species very closely resembling its prey species, which are octocorals, a kind of soft coral. Some of the species are also unusual in that they are able to utilize zooxanthellae from their prey, in a symbiotic relationship that provides them with extra nutrition from photosynthesis, hence they are commonly called "solar-powered" sea slugs.