Zombishrum 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 late May 2020.
Zombishrum could be purchased for $4.99 in a limited time deal which included packages of 8000 coins and 1000 mushrooms. 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: Zombishrum was released again in the January 2021 opening of The Fungus Amongus Planet event. This time Zombishrum could be purchased in a different item deal for $19.99. This limited time item deal included the Screamer skin, 3000 mushrooms, 35000 coins and 2500 DNA.