Mild diarrhea is the passage of a few loose or mushy stools.
Moderate diarrhea gives many watery stools.
The best indicator of the severity of the diarrhea is its frequency. A green stool also points to very rapid passage and moderate to severe diarrhea. The main complication of diarrhea is dehydration from excessive loss of body fluids. It's dehydration you need to worry about, not the presence of diarrhea. Occasionally it is caused by bacteria or parasites.