Wake up with stains-streaks on pillowcase. yellow-rust-greenish light color. usually a few inches long, often 2 tracks home and away. not always. doesn't seem like blood.

Spit: People with acid reflux have night time regurgitation and sometimes find spit stains on the pillow in the A.M. Otherwise, based on the information provided, nothing else comes to mind unless you had wet cough at night that you are not aware of.

Likely snot: It is probably nasal drainage, either directly from your nose, or as post-nasal drip type, via your mouth. Should consult your dr for exam, as color spectrum is not normal.