that is plastic kelp to go along with the plastic rocks in a freshwater octopus tank. hahaha. it is seriously fake though and i have seen people keep it in freshwater tanks before. i service a few tanks that youd swear were salt water until you look at the fish. filled with dead corals and crushed coral gravel. everyone has their own personal taste. and i believe thats at a public aquarium and they have it set up like that for ease of maintenance