This is (probably) "konnyaku" (コンニャク). It is called "konjak" in English according to Wikipedia. Konnyaku is the name of a plant, and this is used to create a starch based food. It is like a very chewy jello. In Japan, quite a lot of food contains konnyaku. It has close to zero calories and the chewiness of it more or less means you lose calories by eating it.
