If it got litigated and I were the judge, I'd be concerned they were trying to abuse trademark to get patent-like protection. In the narrow packaging market, another mushroom packaging competitor would have trouble talking about its product without mentioning the word "mushroom" and drawing the ire of Mushroom™ lawyers.
Disclaimer: lawyer law blah blah
Per the webpage: "The mycelium binds the agricultural waste together, so it can be baked into durable protective packaging"