So its the old fungalganger twist, almost fell for it this time, almost, but I've read Thor vs LuBu so you will have to try harder then that to get me.
I assume there's a chance for the "mushroom" to instead replicate and make a copy. And that one is the one who is aggressively a monster? That's why the guy asked which one was he, but decided it didn't matter. Possibly because there's a chance to still become one? make more copies? still infect others?