Nudibranch

Thanks, @Leuba :)
This one is more common, have seen them many times.

Another one that is similar looking, from the same genus and still not described is a rarer find :

http://www.projectnoah.org/spottings/375...

Incredible !

Thanks, @Ashley for the nomination.

Your spotting has been nominated for the Spotting of the Week. The winner will be chosen by the Project Noah Rangers based on a combination of factors including: uniqueness of the shot, status of the organism (for example, rare or endangered), quality of the information provided in the habitat and description sections. There is a subjective element, of course; the spotting with the highest number of Ranger votes is chosen. Congratulations on being nominated!

Thanks, @Mark :)

That's amazing.

Thanks, @Ava :)

Fantastic!