Ignoring all the fish, I'd say a couple rounds of choking and exposure to a disease that causes you to grow a slime mold amoeba in a way similar to a slaad egg or something. And I guess we need to establish conditions that cause it to release the spores.
According to wikipedia, they can revert to the slug or amoeba form if it gets moist again. So let's say "when mature" or else if the sporocarp takes some amount of damage?
You mean not write the hazard? I think it's worth putting in.
Hmm, considering that the spores are 1 to 2 ft in diameter in the original monster, maybe they're not a good inhaled poison.Let's see if Shade has an opinion before deciding. I like the idea of a hazard, but I'm not too firm on it.