The "face" is just another natural landform, no matter how weirdly shaped. Odd-shaped landforms can be made through wind erosion. Wind erosion can occur on any landform, and the outcome can be any shape. If it was not eroded, the planet could just have a landform different from what we have on Earth. Just because there is an oddly shaped landform on another planet does not mean that it was alien-made.

Much about Mars is undiscovered. This can mean that there are odd landforms which have not yet been discovered. Some of these landforms have the possibility of being shaped like a face or the sillouhette of one. The indentations of the "faces" can also be craters from meteors hitting the landforms. Because much is undiscovered, we must not jump to conclusions.

Natural landforms can take many shapes. They can range anywhere from cone-shaped to face-shaped. Just because it resembles a face does not mean it is alien-made. However, it could have been eroded to look similar to one. Many things are capable of making face-shaped landforms, not just aliens, which we are not yet sure even exist.