This video says about itself:
9 April 2012
This video was taken in shallow water off Green Turtle Cay in the Bahamas. The Unicorn [Leatherjacket] Filefish is normally a deep water fish but is occasionally seen in shallow water. You can see the fish change its coloring as it swims away.
It turns out that a few days before this, another tropical species beached near ‘s-Gravenzande, more to the south in the Netherlands.
It was a unicorn leatherjacket filefish. Also, the first time ever in the North Sea.