Earlier quoted context omitted.
Seriously, is this a threat or is it the unexpected boon of getting a jump-start on terraforming? If you could press a button and "contaminate" Mars with life that would start spreading and generating an oxygen atmosphere, wouldn't you?
It would mean you have a much harder time finding out if Mars once had life, if instead you're detecting mold that hitched a ride with your spacecraft.
Kim Stanley Robinson's Mars Trilogy[0] deals with this a great deal, the early scientists pretty quickly devolve into two camps, terraformers and preservationists. There's a lot of other things going on as well, but this is one subplot.
At one point in the book, a terraformer made the argument if we discovered life on Mars we'd never know how or when it got there anyway. The monkeys in South America are an interesting parallel - total mystery. Best hunch is that some survivable population "rafted" across the Atlantic at some point - under conditions somehow rare enough to have happened exactly once in 40 million years.
KSR doesn't explicitly take a side, but seems to think that terraforming will be inevitable, so we might as well get on with it.
On the other hand, Mars may not be terraformable.[1] The challenges there are so extensive as to possibly be insurmountable. Consider that we are so accustomed to certain environmental conditions that changes so subtle as working extended hours in a popcorn factory can give us cancer.[2] Mars just has toxic soil, a low enough gravity as to probably prevent a sustainable atmosphere, and is constantly bombarded with radiation.
But who knows, futurism is hard because humanity is pretty ingenious.
Isaac Arthur has an extended video on terraforming that's worth watching if you want to dive deeply into this topic.[3]
[0] https://en.wikipedia.org/wiki/Mars_trilogy
[1] https://en.wikipedia.org/wiki/Terraforming_of_Mars#Challenge...
[2] https://en.wikipedia.org/wiki/Bronchiolitis_obliterans#Diace...