Could humans mistake alien life for rocks?

Yes, humans could mistake some forms of alien life for rocks, especially if the life were microscopic, hidden inside stone, mineral-covered, or extremely slow-moving.

This is a real concern in astrobiology because scientists must distinguish between abiotic (non-living geological) and biotic (living) features. Alien life might not resemble Earth life, and in some speculative cases it could have unusual chemistry or mineral-like structures. Very slow metabolism could also make an organism appear inert to human observers or spacecraft instruments.

There is also a known problem with false lookalikes: non-living mineral processes can produce rock or fossil-like shapes that resemble signs of life, and microbes on Earth can live inside rock pores and cracks, making them easy to miss.

So the short answer is yes: it is possible for alien life to be mistaken for rocks, or for rocks to be mistaken for alien life. Whether we could tell the difference would depend on the biology, the environment, and the instruments used to study it.