I found the perfect colour of green yarn (Red Heart Soft Touch to be exact), and so, I have my Halloween costume. I made a brain slug! LET ME SHOW YOU!
I've seen a few patterns online (including a few listed here), but ended up making it up as I went along. 2 strands held together in hdc made up like a kids hat (started with 14dc on the first round & increased each round until 36 total hdc (it seemed wide enough at that point). The last row I used a modified petal pattern & ended up with 5 blobbies. Its stuffed with polyfil, and I made a separate circle in dc to stitch to the bottom to hold it all in. The eyeball is a yo-yo from an old t-shirt with a black button. I did a 4 stitch i-cord for the antenna, tied in a knot at each end. I used what I think is fishing line to attach it to a black headband, which also kind of helped it to mold to the curve of my head a bit.
All in all I'm pretty satisfied with it, and it will serve perfectly well for a work-place costume.