Can't identify + or - in the 5v USB wall adapter, does it matter?
Power (which comes from your wall outlet), is AC, which means alternating current. It changes 50 or 60 times per second from direction, so this is the reason it does not matter if you plug it reversed or not. Some countries have plugs which can be plugged in normally or 180 degrees rotated (but you cannot see what is 0 or 180 degrees since these are symmetrical).

The circuit inside the adapter makes sure the AC is converted to DC, where it matters what is Plus or Minus.