Depending on what you call by aliens. There are aliens who carry a physical body or 3D body, and there are some who can switch from 3D to 4D and more. We call them inter-dimensional entities. The reptilians are in the 3D and can switch it to 4D. So , they can be perceived as so called spirits in their 4D realm but as just another type creature in their 3D. So it all depends.