You guys know how fungus works, right?
It starts with a little spore. Just a tiny speck of dust, like a floating tentacle monster embryo.
If it gets onto a substance it can survive in it grows tiny roots and digs itself in. Not roots like a tree, these roots are like tiny fiberclass organic strands. So thin, they can branch out and surround the surrounding area until there's more thread than meat.
Then they begin to secrete a strong digestive acid, turning the meat around them into a soft pulp; its not an unfair comparison to say it does on the inside what a spider does to the outside while it liquifies the insides of an insect to consume.
The fungus grows until it can release more spores. Sometimes it will passively emit them, like breathing, using a small nub that erupts through the skin of its host; if scratched or damaged it will simply release all the spores it has inside at once, filling the area with its tiny tentacle embryonic offspring. If its tendrils meet another of its kind, the two will merge and become like a single entity; a large growth will erupt where they have met, which will exhale spores in a vast radius. As more and more of these spores are released and meet the surrounding area becomes like a single organism made up of hundreds of thousands of acid-emitting hairs. Once these hairs overtake the host entirely, filling every crevice of its body until there's no more room to move and grow, they expand outwards, making the subject that is far more fungus than its original state resemble a cotton ball, or appropriately enough a creature caught in a spiderweb.
