I'm a stain

What am I? Looking from farther away on the moon. Finally started knowing accurately. I? Just a stain on the moon..... Everyone started comparing myself! My face hiding from my days. My shaking nerves were frozen in those poisonous corridors. Myself separated from the ground not rooted but dropped out of nothing. I'm sad, cold and wet, like a weak tear. In a thin skin don't know far and near. It's useless for me to find any place to hide.