Clypeaster japonicus
タコノマクラ

Known in Japan by the common name of "tako no makura" (octopus pillow), this is a common species in Japan and member of the sand dollar & sea biscuit order of Clypeasteroida. It uses its spines (feet?) to move pieces of seaweed or debris over itself to stay hidden. Typically found at depths between 15-30 meters.

Izu Oshima, Tokyo, Japan