the filmy, moist, dead skin particles, that gets stuck underneath your fingernails when reaching inside your shorts to scratch that crack itch. this substance may perhaps be caused by sweat, forming a cream-like layer that can easily be removed.
bob: man i had the most insane itch in my crack and when i scratched it, there was a bunch of smelly gunk under my nails, i thought it was shit but it was like....soft dead skin..o_O
ned: yah man! looks like you got a case of bad buttcrack-plaque!