Faecium is basically poop. You get this from creatures and sometimes other plants like Gutrot flowers. It’s used for crafting and refining as well as different cooking recipes, as weird as that sounds. 🙂
To get faecium, feed creatures on planets and wait for them to ‘release’ it. It’s fast.
Like I said, you can also find it from the plant and it’s possible to grow it in a lab if you want to keep a steady stockpile of it. 🙂