Originally Posted by dr.lee.baugh
Hard to tell looking at those pictures...depends on how clean you were while doing the fluid change. It is certainly possible what you are seeing is oil from when you took the cover off and re-filled it.
I would clean all the residual oil off the diff, and then keep an eye on it. It may take quite a while before you notice a drop in the diff fluid level, even if it is leaking - a gear oil leak always looks WAY worse than it actually is!
This, ultimately that is a slow enough leak that I would not be worried about leakage in the short term damaging anything. Ultimately as stated it could be residual from the removal process, if you didnt completely clean the cover inside and out well the bottom is the most likely to get hit by stray gear lube as you pop the cover off.
again as stated, get some brake clean and spray it down and keep an eye on it.