Sometimes you may find that some of the plating pads on the LEDs from CreeLED are dirty and look like they are oxidized.
For reference, check this photo:
CreeLED confirms that this happens frequently with CREE LEDs in cut tape quantities and it is not a defect issue.
The backside contacts of the LED are silver plated. The LEDs sit in a pocket with a hole at the bottom of the tape. When the LED bag is opened, the trace amount of sulfur in the air will oxidize the plating of the LED. This is why there is corrosion in the center part of the LED.
Fortunately, flux removes the oxidation when customers reflow solder the LEDs onto boards.