To our current knowledge there is no life on mars.
Regarding the safety of Curiosity's MMRTG:
From NASA - Mars Science Laboratory
Launch Nuclear Safety:
The MMRTG [Multi-Mission Radioisotope Thermoelectric Generator] contains 10.6 pounds
(4.8 kilograms) of plutonium dioxide as the source
of the steady supply of heat used to produce the
onboard electricity and to warm the rover’s systems
during the frigid Martian night.
Like previous generations of this type of electricalpower
generator, the MMRTG is built with several
layers of protective material designed to contain
its plutonium dioxide fuel in a wide range of potential
accidents, verified through impact testing.
In the event of a launch accident, it is unlikely that any plutonium
would be released or that anyone would be exposed
to nuclear material.
The type of plutonium used in a radioisotope
power system is different from the material used in
weapons, and cannot explode like a bomb. It is manufactured
in a ceramic form that does not become a significant
health hazard unless it becomes broken into very fine pieces
or vaporized and then inhaled or swallowed.
who might be exposed in a Mars Science Laboratory launch
accident would receive an average dose of 5-10 millirem,
equal to about a week of background radiation.
American receives 360 millirem of radiation each year from
natural sources, such as radon and cosmic rays.
Curiosity's MMRTG uses plutonium-238 dioxide as its nuclear fuel, which is an alpha emitter.
From Princeton University - Environmental Health & Safety
Because alpha particles are charged and relatively heavy, they interact intensely with atoms in materials they encounter, giving up their energy over a very short range.
In air, their travel distances are limited to no more than a few centimeters. As shown in the following illustration, alpha particles are easily shielded against and can be stopped by a single sheet of paper.
Since alpha particles cannot penetrate the dead layer of the skin, they do not present a hazard from exposure external to the body.
However, due to the very large number of ionizations they produce in a very short distance, alpha emitters can present a serious hazard when they are in close proximity to cells ...
Special precautions [should be] taken to ensure that alpha emitters are not inhaled, ingested or injected.
- Curiosity's MMRTG contains 4.8kg of Plutonium-238
- The MMRTG has several layers of protective material designed to contain the plutonium in case of a wide range of potential accidents
- Plutonium 238 emits alpha particles
- Alpha particles have a very short range and skin is enough to shield against it
- However, alpha particles can pose a danger to "naked" cells
How martian life would be affected by a damaged MMRTG depends on what kind of life it is. And currently we can only speculate about that, since no life has yet been found.