The concern isn't Earth organisms evolving and attacking us.
The concern is they'll make it harder to detect potential life on other planets (because of false positives) or even that they'll destroy alien life if it exists.
Yeah, which is definitely a concern for a planet with the potential to host life, like Mars or Europa, but for a place where we are pretty much 100% confident life cannot exist, like the Moon, it's just not a concern. Tardigrades are tough little creatures, but even they will just enter a tun state in such a harsh environment (if they even survive).
