This is a sea slug- if the word ‘slug’ makes you curl your lip- try ‘warty slug’ on for size. Delicious.
These animals graze on algae and sometimes different types of stinging hydroids and then transfer the poison to their skin, making them fairly unappetising for fish. Like many animals on land, they colour themselves brightly to advertise the fact that they have a toxin. Which is a good thing because being a slug, they can’t exactly flee at the first signs of danger.
Photographers love slugs and nudibranchs for these reasons: they’re colourful and they don’t move. So even amateurs like me can get a decent focus!