Terran life is voracious and competitive and the result of 3-4 billions of years where only the winners get to carry on to the next generation. The life that evolved here on Earth is far more likely to destroy other planetary ecosystems than the other way around.
Obviously, precautions are warranted as we are dealing with something unknown. As you could end up with some annoying consequences like space mold eating your hatch seals. But as for some alien life showing up on Earth and taking over? If it's possible at all the probability is incredibly low, based on our understanding of life and evolution. Good luck to the martian lichen trying to pry native species out of their habitat.
And we have already traded rocks with other planets. And we've shown that some forms of life can live dormant in space for a very long time. If there is life on Mars, it is possible it's already been to Earth, and was immediately gobbled up.