Note 1: The "UFO" appears in DSC_1604 but not in photos taken one minute earlier or later so must either be something in the Earth's atmosphere or some artifact "noise". The Nikon D50 Guide states that "noise" may occur in exposure speeds over 1 second in the form of random-spaced brightly-coloured pixels. However the shutter speed here was 1/8 second, the pixels non-random (an even circle) and are are dark as shown to the right.
I suspect it was a low flying earth satellite, as this would have a rapid "transit of the Moon" and show up against the lighter moon as this does.
Note 2: The "UFO" is seen just North-west of Mare Nectaris
Note 3: As we are in the Southern Hemisphere, turn your map of the Moon upside down and west is now to the right and north is down! If you don't have a Lunar Atlas look at the NASA's Major Lunar Mare (It's easier to print it then turn it up side down).