It is almost always necessary to manually focus astronomical subjects. The AF systems in most cameras can't focus on small, dim objects in the sky. Even when they can, their margin of error is usually too great to give the kinds of results most people desire when doing astrophotography. The same is true of focus markings on lenses that have them - they're not accurate enough for critical work with point sources of light such as stars.

Because infinity focus changes with focal length and other environmental factors, most modern lenses allow the focusing elements to go past infinity. Many lenses with high speed AF motors allow an even greater buffer past infinity focus so the focus motor is less likely to bump against the end of travel when trying to focus at infinity.

With digital cameras manual focus using the viewfinder can be difficult even in bright light. It's even harder in dim light. Fortunately there is a better way if your camera has a Live View shooting mode. Set the lens to "manual focusing", use the Live view magnifier to zoom in on a section of the sky (in your case the moon) and manually focus until it is as sharp as you can get it. Since AF is turned off, the lens will stay focused at that distance as long as the focus ring is not moved. You can then exit live view and shoot using the viewfinder to compose. Just be careful not to touch or move the focus or zoom rings on the lens.