Realistic alien costume ideas include using a balloon to stretch the mask and make it appear realistically flesh-toned. For the body, wear a zentai suit or a body stocking to create an alien skin appearance.
The body stocking suit and balloon mask work best together if they are similar colors. This simulates a uniform skin tone for the alien. Common colors for the skin and mask combination include green, gray and blue. To make the eyes, use sunglasses that are shaped appropriately for the outfit, though they can cover up the lenses to create the desired shape underneath the mask.