Believe it or not but this is a sea cucumber. To be more accurate: a juvenile Blackspotted sea cucumber (Pearsonothuria graeffei). This beautiful little thing mimics a Phyllidia nudibranch in order to be thought equally distasteful by potential predators. Interestingly, the less colourful adult form of this species possesses toxins that do make it unpalatable to eat. Juveniles however lack this kind of defense and are forced to be resourceful in order to stay alive long enough to develop this chemical protection.
When a completely harmless species mimics a toxic or distasteful species it is called Batesian mimicry, of which this juvenile sea cucumber is a great example!