Cootie is one of the three skins released in The Fungus Amongus Planet event. It could be purchased in the item deal part of that event in early September 2020.
Cootie could be purchased for $14.99 in a limited time deal which included packages of 3000 mushrooms, 12000 coins and 600 DNA. 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.
Note: Cootie was released again in the May 2021 opening of The Fungus Amongus Planet event. This time Cootie could be purchased in a different item deal for $9.99. This limited time item deal included the Mari Monshroom skin, 1500 mushrooms, 13000 coins and 1000 DNA.