Sporocarp (Ex): Protiston can form its body into sporocarps -- objects that resemble mushrooms about 10 feet tall -- as a full round action. It can use its Self-Division ability to form any part of its body into a sporocarps. A sporocarp has the same spellcasting ability as an equivalently sized slug, and it gains Automatic Silent Spell (x3) and Automatic Still Spell (x3) as bonus feats in sporocarp form.
Partially developed sporocarps are soggy and covered with damp mucus, giving them fire resistance X. Such a sporocarp does not have mature spores or a spore attack.
A mature sporocarp is dried-out and has vulnerability to fire. The head of the "mushroom" is hollow and filled with spores about an inch long. It is AC 1, hardness XX, and takes XXX 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 and attach themselves to any organic matter they touch (including living creatures), which they immediately start to digest, doing XdX points of acid damage per round (the acid damage only harms organic materials such as flesh or wood). On the first round of contact, the spores can be scraped off a creature (exposing the scraping device to the spores' acid damage), but after that it must be frozen, burned, or cut away (dealing damage to the victim as well). Anything that deals cold or fire damage, sunlight, or a remove disease spell destroys a spore infestation.
Given enough time and food, Protiston can regrow its entire body from even a single spore.
To fill in the blanks, the original (from standard slime molds) sporocarp had fire resistance 5 in soggy form. In hard form, it has AC 1, hardness 5, and takes 120 hit points to destroy. The spores do 2d4 acid per round. Want to boost those any?