How about this?
Sporocarp (Ex): Protiston can form its body into sporocarps -- objects that resemble mushrooms -- 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. The sporocarp is twice as tall as the reach of the equivalently sized Protiston slug.
Partially developed sporocarps are soggy and covered with damp mucus, giving them fire resistance 10. 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 10. The sporocarp has as many hit points as an equivalently sized slug, 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 acid damage equal to an equivalently sized slug's slam 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.
I'd change the references to the "slug", since our working draft doesn't use that term for Protiston's normal form.
We'll also need to say how long it takes for a sporocarp to mature - maybe a minute or a few rounds?
I'd also have the size of the spore cloud vary with the Sporocarp's size.
Revising...
Sporocarp (Ex): As a full round action, protiston can form its body into sporocarps — objects that resemble mushrooms. It can use its Self-Division ability to form a part or parts of its body into sporocarps while the rest of its body remains in its normal slime form.
A sporocarp has AC 1, hardness 10, and the same hit points and spellcasting ability as a normal piece of Protiston and it gains Automatic Silent Spell (x3) and Automatic Still Spell (x3) as bonus feats while in sporocarp form. It is incapable of moving (so has speed 0 ft. and cannot make physical attacks), but has full use of its senses and mental abilities. A sporocarp is twice as tall as the reach of the equivalently sized piece of Protiston.
Newly formed sporocarps are soggy and covered with damp mucus, giving them fire resistance 10. After X rounds the sporocarp will mature, drying out and gaining vulnerability to fire.
The "mushroom" head of a mature sporocarp is hollow and filled with spores about an inch long. Any weapon damage which does not destroy a mature sporocarp causes it to split open and release its spores. The spores scatter in a cloud covering a radius equal to 30 foot or the sporocarp's height, whichever is greater. The spores in this cloud attach themselves to any organic matter they touch (including living creatures), which they immediately start to digest, doing acid damage equal to an equivalently sized piece of protiston (see Self-Division, above). This 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.
The spores will form protiston cells and eventually regrow into squares of protiston's slime-body, a process that usually takes X minutes? hours? days? A single spore can reform Protiston's entire body in as little as Y days if it has sufficient food.