Fucus-like sea-horse. "Fucus" is a genus of brown algae...it's not like a slimy algae you'd find in a pool, but what most people call a "seaweed". Many seahorses that live in cooler environs are shaped and colored similarly to endemic plant life. More tropical seahorses tend to be the more brightly colored ones, just like other bony fishes.
The Royal Natural History. Richard Lydekker, 1897.