This is thought to be a pixel artifact, nothing more. Google Mars is hardly the best source to find "stuff" on Mars. If you want to verify this, all these images are available in their original state, the fun part is finding them, since there are so many (and so many sources).
It should be known that once the origin source photo is found, or a photo from another angle, it never turns out to be what people think. I remember one photo people had colored in, and were POSITIVE it was a "base" on Mars. Then, someone dug up the photo from a bit of a different angle - it turned out to be nothing at all, just a depression in some rocks.
Keep in mind that these photos are "sewn' together out of many images, and pixel anomalies are pretty common, especially when using a source like Google.