Green Monster is a 2D platformer game where the main character is a green monster. The game has two modes: Levels and Modes. In the Levels mode, the main character tries to complete 3 challenging levels by jumping on platforms, avoiding fires, and collecting coins. The second level is similar to the first one but with additional features like teleporting blocks, moving platforms, and jumping blocks. In the third level, the character is in a completely different environment and has a weapon to kill all the enemies and defeat the boss.

In the Modes section, there are two modes: Endless and Battle. In the Endless mode, the character jumps on platforms, avoids fires and falling blocks, and tries to go as far as possible. In this mode, the character collects coins that can be used to buy items from the shop. In the Battle mode, the main goal is to kill as many monsters as possible. The monsters get faster as time goes on, and after killing 50 monsters, the character gets a short time to collect coins. These coins can also be used in the shop.

The game also includes a costume shop, an item shop, and an achievements section. The achievements section displays the player's highest score and most kills. In the costume shop, players can buy new costumes for the character. In the item shop, players can buy items like health, ammo, and weapons for the character to use in the Battle mode.