A little bit does a lot of warming. Second, PFCs have a very long lifetime. This causes serious problems on Earth, but their longevity would be a positive factor on Mars. Third, they do not have any negative effects on living organisms. On Mars, building up an ozone layer in the atmosphere would be an important goal of terraformers.
Greenhouse gases prevent that energy from escaping into space.
The sunlight that hits a planet's surface arrives primarily as visible and ultraviolet light. The planet absorbs this solar energy, and then re-radiates warming infrared energy back out into the atmosphere. Greenhouse gases in the atmosphere work as a global layer of insulation, trapping that infrared radiation and preventing it from escaping into space. CO 2 and water are good at trapping some of this infrared energy, but not all of it. On Earth, there's so much CO 2 and water in the atmosphere that it doesn't matter if some infrared radiation escapes back into space.