Marimo pet. It's a live green algae, which naturally forms a cute round shape. It will grow bigger slowly over time. It gets its name from Mari meaning "ball" in Japanese and "mo" meaning algae in Japanese. Japanese people consider Marimo ball as a National Treasure and believe that it will bring good luck, so it's very popular in Japan.
Luck in a Bottle - Marimo Moss Ball Aquatic Terrarium