In the screen shot I have my mouse over a tile that contains an egg shell, algae, slime, more slime, and polluted water.
I'm in the material's overlay and have selected organics. If a wad of slime is sitting in a tile, this will normally highlight the thing in icky green (good choice). As you can see, nothing is highlighted.
Everything in the tile except the polluted water is organic.
Egg shells don't show up in the organic display even when they are alone on a tile.
So your overlay has a few problems with organics it appears.
Steps to Reproduce
This issue has not been confirmed by a developer yet.