It's abnormal especially if it's greenish or yellow in colour and has a bad smell. I advice that you take the child to see a doctor for proper evaluation and management.

It's mostly likely she had a dream or
nightmare especially if she has watched
something scary or frightening. No
need to panic but just calm her down and
gently reassure. It will surely pass.
Always ensure kids are kept away from
things that can scare them or frightening
events or TV shows.