Black-tipped Spiny Dorid (Acanthodoris rhodoceras), 16 July 2019. We've seen a few individuals of this nudibranch recently, all with this very dark coloration. Note the two long rhinophores tipped with reddish-brown, and the prominent gill plume (it looks a bit like a flower). There are also nice gold highlights along the edge of the mantle. Thanks to Eric for spotting it!
P.S. Interestingly, when we were seeing this species several years ago (in 2015), most of the individuals we found were much paler. For an example (with video), see the post called "A few tips" on 6 June 2015.