Reticulidia fungia  Brunckhorst & Gosliner in Brunckhorst, 1993

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Reticulidia fungia is characterized by yellow or pale ridges which have a fine white border and by its broad blue-grey mantle margin. Pretty similar is Reticulidia halgerda which has numerous, narrow yellow ridges and an orange mantle margin.
More information on Reticulidia fungia are on Bill Rudman's Sea-Slug Forum!