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Very similar is Goniobranchus setoensis (Baba, 1938) [Glossodoris] which differs by the lack of purple spots in the greyish mid-dorsal region.
The Sea-Slug Forum lists both species as Chromodoris decora and dicusses the colour variations at Identity of Chromodoris decora , let me quote:"..all these forms seem to represent variations of one species." and "Baba himself (1953) suggested it should be considered a synonym of C. decora."
IPN lists them seperate with the comment: "Additional work is necessary to determine whether these are color variants or distinct species."
Scott and Jeanette Johnson discuss it at Underwater Kwajalein with many pictures, they think these species to be distinct.
More information on Goniobranchus decorus as Chromodoris decora are on Bill Rudman's Sea-Slug Forum!