This Spanish Dancer - Hexabranchus sanguineus is a very large Nudibranch, around 30 cm.
Usually, Nudibranch are slow crawlers on the sea bottom but when disturbed they can swim in the water column and when do that, the mantle opens up making it look like the skirt of flamenco dancer, hence its name.
A video of a similar Spanish Dancer swimming can be seen in this video of me with a Spanish Dancer in Philippines :
This was seen in very deep waters of around 40 meters during a dive in Mozambique.
The Spanish dancer, scientific name ''Hexabranchus sanguineus'' , is a dorid nudibranch, a very large and colorful sea slug, a marine gastropod mollusk in the family Hexabranchidae.