There is no way earth microbes can destroy the Mars ecosystem since both will have the same microorganisms. It is unlikely that a Earth rock landing on Mars now

There is no way earth microbes can destroy the Mars ecosystem since both will have the same microorganisms. It is unlikely that a Earth rock landing on Mars now will land somewhere productive, but in the past when Mars had surface water it was much more likely. The really interesting thing is we know so little about the micro-organisms here on Earth that we would not even be able to tell if we find a new phylum of bacteria or archea on Mars if it was native or not.