Phyllidia and Flatworms
Phyllidia don't have the prominent feathery tuft of gills, and often have lumpy ridges on their backs.
Flatworms are... well, flat.
These have similar colours but they've probably each got their own name.
Click here for more black and white phyllidia
Phyllidia Ocellata (or ocellated wart slug)
Think this is some kind of halgerda, which belongs to the phyllidia family too.