It looks like abrasion to me. It could be from another fish's mouth if they were fighting/chasing. It could also be from the fish scraping against something in the tank. Sometimes they'll 'flick' themselves off of the substrate, rocks, or side of the tank for a variety of reasons. So long as there aren't visible parasites, and I don't see anything, I wouldn't worry about it unless they start doing it constantly.
What to do? In time it should heal. The only thing would to be watch for any sign of secondary infection. The primary concern would by fungus, which would look be white and like cotton to some extent (fluffy, puffy, etc). Just be sure to keep the water in good shape (keep up on your water changes, clean the mechanical filtration like normal, etc) and you should be fine. Healthy fish are rather resilient.
