Based out Stout et al. 2010, who found morphological and genetic differences between Pacific specimens and Arctic/Atlantic ones, and thus recommended that all Pacific D. frondosus be considered D. venustus, using the name originally assigned to them by MacFarland.

So,

1. If your slug is in the Atlantic, it is http://www.inaturalist.org/taxa/423919-Dendronotus-frondosus
2. If your slug is in the Pacific, it is http://www.inaturalist.org/taxa/129995-Dendronotus-venustus
3. If your slug is in the Arctic, it is probably http://www.inaturalist.org/taxa/423919-Dendronotus-frondosus, but as the authors point out, "The range of D. venustus is unknown and more species from the Arctic need to be examined."

The authors also found that Atlantic specimens were genetically nested within the Pacific ones, suggesting there may be multiple Pacific species to consider, but thankfully they didn't go so far as to split it further.

Note that this does seem to be the accepted treatment in WoRMS: http://www.marinespecies.org/aphia.php?p=sourcedetails&id=147622