Holey Sponge Seaweed (Ceratodictyon spongiosum) カイメンソウ
Holey Sponge Seaweed is red algae although it is often green often very dark with noticeable pores. It is also symbiotic with the sponge (Haliclona cymaeformis) which explains the spongey texture. Amami 2022
Holey Sponge Seaweed Amami 2018