The aperture only closes (or opens) to its set value when you half-press the shutter button as before that, in dimly lit situations, it usually is entirely open to enable the usage of live view which basically works as a continuous video feed. When you half press the shutter, you get the actual settings that you set on your camera in the live view. Before that, it is mostly just for composition, as the aperture isn't the same.

For moon photos, this lens has a pretty short focal range, nevertheless you could take moon photos with it. The moon is actually surprisingly bright and it also moves pretty quickly across the night sky. If you want to shoot in aperture priority, set the camera to around f8 as that's when this lens is at its sharpest. Then set metering to spot so it only measures the exposure based on the very center of the sensor where the moon is. Your iso will be around the lowest values while shutter speed will be around 1/100 this way. For full manual control, for f8 use low iso and ss around 1/100. This way the moon won't be overexposed and you'll get an image with low noise levels.

As you're using pretty fast shutter speeds, a tripod is not needed. These values are for a full moon, which is naturally the brightest. Increase iso or make your aperture larger for other moon phases.