That's ok, the one you have posted is good enough for identification. This is an undescribed Gymnodoris, i.e. it has not been given a formal species binominal. But it looks a lot like the described Gymnodoris nigricolor, but different enough to not be the same animal. So it is acceptable in biology to call an undescribed species using the genus - here Gymnodoris - and the term sp. or if it is similar to another cf. or aff. something. Cf meaning 'confer' (to compare) and aff. meaning 'affinis' (similar to). For a much clearer explanation have a look at this page in Wikipedia https://en.wikipedia.org/wiki/Open_nomenclature