Thuridilla hopei is a species of algae-eating sea slug occurring in the Mediterranean. In nature it can mainly be found close to its preferred food source, the green algae Cladophora.
Cladophora species can be maintained quite well in aquaria - thus this beautiful little sea slug might be a nice addition to Mediterranean biotope tanks. We don't know much about the longevity of the species or if it reproduces in captivity.
The image of the specimen above has been shot by Tobias (username Acanthoscurria) in the Egyptian Red Sea. T. hopei usually only occurs in the Mediterranean but it would not be the first species that crosses the border between those two seas.