Here's a diatom (alga in a glass case), probably Cymbella sp. (apparently also called "rock snot"...), sitting atop a stalk of mucilage. These diatoms can sometimes be seen on rocks in creeks and streams as fuzzy brown stuff growing, comprised of large colonies. In masses, they are also extremely slimy -- perhaps you have unknowingly received damage from these critters making you slip and fall. The brown stuff is the plastid itself, taking up much of the cell -- you can almost make out the thylakoid stacks as fine striations on the top part, where the plastid curls towards you, providing us with a handy cross-section. Energy is stored in the fat globules seen in the middle of the cell. The very centre contains the nucleus, which is not visible in this section. Diatoms are much thicker than we like to imagine them, sometimes thicker than they are wide. Surrounding the frustule (glass case) is more mucilage, seen as that faint outer contour. This probably is what makes their masses exceptionally slimy -- you're stepping on layers and layers of glass cases embedded in balls of mucus. In other words, avoid stones covered in brown fuzz when crossing rivers -- to prevent crushing innocent colonies, of course, rather than anything to do with self-preservation.
The views expressed are those of the author(s) and are not necessarily those of Scientific American.