It's his tongue. Octopuses taste through their limbs. Their limbs combined contain over half of their nervous system and seem to move independently of their brain - their brain doesn't seem to control their arms all the time, and instead the arms independently explore the world around them, feeling and tasting what's there. But the arms can be brought under control, such as when the place them into a very specific torpedo configuration to hydrodynamically move quickly through the water. Their brain is very structurally different than mammalian brains, despite having many of those different structures having the same functions as found in mammals. Their brain is like a donut around the oesophagus and two hemisphere-like structures that aren't connected. Eating sharp objects can be fatal to an Octopus, due to part of their brain surrounding the oesophagus.