This is a Cuthona caerulea video.
Translated from the Dutch marine biologists of Stichting ANEMOON:
Sunday, September 14th, 2014
Recently, on the North Sea beach of Walcheren, dozens of Cuthona caerulea, a stunningly beautiful sea slug, were discovered. Previously, up to now only a few specimens had been found in the southwestern Oosterschelde estuary. One of the reasons why so many Cuthona caerulea may occur locally here is the presence of the specific food of these slugs: not yet found elsewhere on our coast, the hydro polyp Sertularella elisii.