You're conflating life as hosts and parasites. Most parasites require co-evolution with hosts. In the case of pandemic viruses, none existing anywhere else in the universe have the slightest chance of infecting us because there is absolutely nothing evolved exactly like us anywhere else.

Or a Martian microbe might just eat through any Earthly thing unable to fight back. Gray goo

Mars... no, obviously that would have to be "red goo"

So it's incorporating large amounts of iron into its system? Take out the magnets and mess that thing up.

Astronauts are routinely quarantined:

https://en.wikipedia.org/wiki/Mobile_quarantine_facility

You realize that the apollo missions were done like 50 years ago, right?

You are absolutely correct that we have not returned to the Moon in 50 years.

They were quarantined in case they bring back some unknown pathogens we don't know what do with.

There are plenty of things that didn’t evolve to harm us but which can nonetheless harm us. Meteors are an obvious macroscopic example, while toxic chemicals are an obvious microscopic example.

I understand that the boundaries are blurry but this particular comment was talking about life harming other life.

Right, and my point is simply that if inorganic chemistry is more than capable of harming life, it’s a bit strange to assume that organic chemistry and life would somehow have less capability to do so.