Couple of possible causes. Might just be a scrape from rubbing against the rocks at night while sleeping. They sometimes wedge themselves in rocks or equipment. Usually heals by itself.
And then there is this condition "Tang Fingerprint Disease" that is a very curious condition. Not much you can do for it. Link with more info below.
Keep an eye on the area to see if it grows, becomes white and filmy, redness develops, or necrosis occurs.
Wanted to start a thread on this, as a member of my local forum recently had a brush with this elusive condition/disease (photos below). I myself have had no experience with it, so everything I know comes from anecdotal evidence obtained online. Probably the best description of Tang Fingerprint...