I would say something quickly, quietly and discreetly. If the guy doesn't fix it or reacts in any way that indicates that it was intentional, then I would look for a staff member and relate it, stressing the reaction that made me uneasy.
This way, if it was innocent, then it was sorted out quickly and with the least embarrassment. If it wasn't, then the staff will also have that information, as intentional flashers may act all innocent if someone in authority calls them out.