The two main causes are physical injury from bumping into something/fighting or parasites such as flukes. For a simple mechanical injury, just watch the fish close for a couple of days to see if it gets any better on it's own. If it doesn't start getting any better within a couple of days, or shows any signs of getting worse, you will need to remove the fish to a QT and treat with antibiotics. In the case of parasites, have you added any new fish in the past few weeks or so?