Spores
I found a mushroom during the zoo mushroom class, it seemed to be a small Amanita--the whitish gills looked mostly free, it had ball-like base, a ring on the stem, it was growing from the ground not from wood. I cut the cap off and put it on a piece of paper and covered it with a small bowl. Then I forgot about it over the weekend.
On Monday I found it--the cap had mostly dried up, but had left this deposit of white spores (Amanitas have white spores, but so do many kinds of mushrooms). The brown discoloration is from the moisture of the mushroom causing a secondary rot of some kind--mold or bacterial. For some reason photos taken with my phone are super magenta, so I have to adjust them way toward green for them to look right.