I would be amazed if we found anything living on Mars, and if it is
alive, it is likely deep underground, far below where we could reach
at present or in the near future.

Plus, would an alien life form actually be able to interact with our
biology such that it could cause harm? I think organisms originating
and developing on two different worlds would not have the mechanisms
for any kind of meaningful interaction, SF monsters aside.

I would hate to see Mars exploration, or exploration of the rest of
the Sol system, hampered by these fears. Though I would like to
be proven wrong in this case, I think we will have to search other
solar systems to find alien life. While conditions on some worlds
in this system have their merits for at least simple alien creatures,
that does not mean they actually exist there now. I think most
extraterrestrial environments are too harsh for any life to get a
foothold on them.