This is what happens to a lot of people's images of the moon. It's a result of trying to get a shot with the camera on full auto. What's happening is that the camera's light meter is taking readings across the entire scene, then it tries to balance it out. Leaving you with an overexposed moon. Here the camera selected F-5.6 at 1/30th of a second with an ISO of 3200 (which is why it appears grainy).
Instead, try using full manual mode. By taking control of the settings, I was able to properly expose just the moon. To do this, I changed my metering mode to Spot Metering. I also set my ISO to 400 to increase my shutter speed and reduce the chance of camera shake. I stuck with an aperture of F-5.6 which gave me a shutter speed of 1/500th of a second. Also, with this speed, there is no need for a tripod.