You may find substantial, dense webs on fungi in autumn. These look quite like the tent-webs of some moth and sawfly larvae, but differ in occurring later in the year; enclosing a fungus instead of a food-plant; not being full of specks of caterpillar dung; you also rarely see any living (or dead) creatures, eggs or pupae in them.
The cleanness of the webs and the way they are restricted to a particular fungus might make you think you are looking at a grey, fuzzy mould.
To find out what was making these, I took a piece home and kept it in a vivarium. Checking on the vivarium in the middle of the night I spotted a small mealworm-like creature moving through the web, leaving a slime trail behind it. I caught it and put it in an empty enclosure where it soon started fervently weaving by bobbing its head back and forth at an impressive rate, extruding silk from spinnerets on its snout. In the morning, the new enclosure was full of webs, and the larva resting in the middle.
I believe these larvae are fungus gnats of the family Mycetophilidae - a large family of small, rather plain flies with thousands of described species, most of which lay their eggs on fungi. I haven't found much witten about the function of the webs: maintaining a humid atmosphere; excluding other small creatures; catching spores for consumption have all been mooted. The larvae also seem to be able to move smoothly and rapidly through their silk network.
