That represents one unhappy citizen. If you surpass the happiness limit (or "happy faces"), then each population point you gain after that point will be "unhappy". The icon represents a population point that is basically worthless until you increase your happiness limit. If you increase your happiness limit, that icon will disappear and your population point won't be wasted.
Another option is to adopt the slavery civic and decrease your population whenever you run into unhappy citizens. Because each population point adds another unhappiness point, the unhappy citizen will disappear.
