Hmmmm. Height of sporocarp=space of slug? Bump hardness and fire resistance to 10? Acid per round = slam acid damage? I could do that.
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.
If we want the new sporocarp to dry quickly, I'd say 1 hour. If we want it to dry during the course of combat, I'd say 1d10 rounds.
Let's say the regrowth time takes 1d4 hours for a 5 ft sq "slug" and perhaps 2d4 days for the whole thing. Sound reasonable?
So did you want 1 hour or 1d10 rounds for the drying time?
Well, the 1d4 hours for 5 ft sq vs 2d4 days for the whole thing was an attempt to interpolate between the linear and geometric growth without getting into the math much. If you want 1d10+10 days, though, that's ok.
Sure, that works for me. But I think "Protiston's" near the end should be capitalized. Are we done after that?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.