Diver Corne Bolders from the Netherlands, who made this video, writes about it:
19 August 2014
During a night dive at the Bergse diepsluis I had a short but very special experience with this at least 15 cm [long] seahorse. In the light of my buddy (Rene Weterings)’s dive light I suddenly saw the silhouette appear. After about 15 minutes he was gone in the night just as quickly as he came. Unfortunately, the seahorse apparently is moving and we have [not] seen him anymore.
This is a short-snouted seahorse.