This video from Zeeland province in the Netherlands says about itself (translated):
March 22 2016
This brightly colored slug is Polycera quadrilineata. Not much larger than 5 centimeters. Despite the strong currents of the Eastern Scheldt, it crawls quietly. The bryozoans it eats need the flow of the tide-water. Therefore Polycera quadrilineata can also be found there. Filmed by [diver] Mirjam van der Sanden in the Oosterschelde.