Green Grub is one of the three skins released in Nomnomnom Planet event. It could be purchased in the item deal part of that event in mid November 2020.
Green Grub could be purchased for $9.99 in a limited time deal which included a package of 15000 coins, 1200 DNA and 3000 food magnets. It was not available for individual purchase and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.