Why not just 2d4 acid damage which doesn't continue or need to be scraped off? And no more spore infestation. Like this:
A mature sporocarp resembles a large, dried out mushroom about 10 feet tall. The head of the "mushroom" is hollow and filled with spores about an inch long. A dry sporocarp has vulnerability to fire. It is AC 1, hardness 5, and takes 120 hit points of damage to destroy, but any weapon damage which does not destroy the sporocarp causes it to split open and release its spores. The spores scatter in a cloud covering a 30 foot radius, doing 2d4 points of acid damage to any organic matter they touch, including living creatures.
Given enough time and food, even a single spore can grow and multiply into an entire silver slime colony. Partially developed sporocarps are soggy and covered with damp mucus, giving them fire resistance 5. Such a sporocarp poses no threat, since its spores are not ready to reproduce.
The only other thing to fiddle with is the size, I guess, but I'd be happy to leave that alone.