It can be hard to get excited about oozes—“Okay, so what’s this one immune to?”—but slime molds actually have a lot of potential, especially because they are loosely based off of real and thoroughly weird, category-defying organisms.
In game terms, the most interesting thing about slime mold is the disease it spreads, enervating its victims and melting away their flesh. Also, there’s adventure potential in those creatures that find slime mold tasty—PCs could be halfway through their battle with one of these oozes when giant beetles or wasps or rat swarms show up. And since many of these creatures also arrive with ability-sapping diseases/stings/venom of their own, they can combo with the slime mold for a surprisingly challenging encounter.
And let’s not forget to look at real slime molds for inspiration. Show your PCs videos of slime molds so they can see what their up against. Also, slime molds are so hard to categorize there’s still debate about what phylum they belong to…so you can allow them to defy categorization (in other words, cheat) in your campaigns as well. For instance, why not use your GM powers to rule that slime molds are as much plant as ooze, allowing your NPC druids to use them as allies and spies…?
Finally, real-life slime molds develop surprising emergent behaviors in response to stimuli. They can literally navigate mazes and form strange networks in order to obtain food and cope with obstacles. So there’s nothing saying you can’t have your slime molds react, learn, and even attack like monsters far smarter than they are. The base model may be Int —, Wis 1, but after an infestation has survived an attack or two, those numbers might well creep up…
Adventurers stumble into a minotaur’s maze. The minotaur is long dead but other inhabitants have moved in, including an infestation of slime molds. The oozes seem mindless at first, but after only one or two encounters the remaining molds seem to become more cagey and aggressive, even appearing to set up ambushes. But surely that’s impossible…
A sleeping beauty waits in a tower walled off by vicious thorns. So the legend goes…but if there was any magic maintaining the reclining princess’s sleep, it did nothing to protect her from the ravages of nature. Any cavalier foolish enough to try to kiss her awake will find that the sheet covering her is a fungal mat, her veil is a lacy mushroom cap, and her bed is a slime mold that lurches to attack.
Long forgotten by its master, a fungus leshy still tends its patch of mushroom garden. In fact, it tended the garden too well—the patch is now a slime mold that slurps through the mossy caverns below Ashwood. The devoted leshy doesn’t understand that its beloved garden is now a predator, and it reacts to anyone attacking the slime mold (even in self-defense) with violence.
—Pathfinder Bestiary 2 249
If you’re looking for the slicer beetle, it’s hiding out with the goliath stag beetle.
Also, once again I’m behind on my mail and comment replies. Bear with me; real life has been a thing.