How big is the Stars and Stripes (I know you said small but even 8" or 10" is small for a S&S
)? Looks like it COULD be a bite mark but hard to tell. Any aggression? Some puffers will go for the eyes when fighting.
A 135G is way too small for a Stars and Stripes (pushing it for a porc really), so hopefully he is going to at least a 200G eventually or you may have problems.
If you haven't seen aggression then it could be some kind of infection. Is the eye cloudy at all? Doesn't look like it from the picture I guess. Are you sure its not just his coloring? How long have you had him?