I was investigating Hartstongue Fern for micromoth activity today, and I was moderately successful, finding some inconclusive micromoth damage to some spore sori (the moth larva had left the scene, so no id was possible).
I did find, however, the following item growing between two sori:
Instinct tells me it's fungal. I then examined it more closely and found that the white fungus is surrounding what looks like the corpse of a spider:
I brought it back to the study, so I'll get it under magnification tomorrow. Early research shows that there are some species of Entomophthora that attack spiders, so I'll check that line of reasoning, too.
(The spider is the pink bit with the black dots.)
The fungus is about 6 mm. across.