when it gets hot u have hot air coming out when it gets cold u have air going back in. (venting)
when the air is going out u might have a little mist mixed in. it does not look that bad to me.
if it was cold when u filled and then the lube expands it will kick it out
for example putting a much deeper trans pan on will add a couple of quarts and when they start up a grade it will dump a quart out of the vent. i have also seen when people put large tires on their 4x4 and run the freeway it will kick atf out the overflow because it creates more heat. on those i recommend just filling to the add mark.
i suppose that the type of lube might have a bearing on the situation. i use 75-90w valvoline full synthetic.
i would not tear it apart for that small amount of vapor . u might get u a heat gun and check temps on that diff and rear wheel so as to make sure u do not have a problem diff, bearings, and brake.
my point is ,,, i forgot