The article doesn’t say if the creature is still or was ever alive while in his custody. If alive, he can see what it does to feed itself. It looks like some form of plant life to me. Think of the Venus Fly Trap, which gives direct reaction to it’s environment. At first glance, one is not sure if they should kill it and cook it or throw it back into the sea, and allow Neptune to reclaim one of his servants.
Some kinda Starfish?
Click on youtube above