Silly Martian ghosts
Why didn't they just kill their host and move on to the next body until every "invader" was dead xP
"Love is gender-less, sex is for procreation. Miauw run off and have a lovely time."
If their goal was to just kill all invaders then you may have a point. But I don't think it's clear what their goals were. Maybe they ultimately want to rebuild Mars to its former civilization. They would need host bodies for that.
Or maybe they felt they could accomplish more using a body. Remember that these ghosts couldn't go through solid objects like walls or doors. That's a pretty big disadvantage as a ghost.
Maybe these martians weren't that intelligent of a race to begin with. We got a glimpse of their species when Melanie was tripping out and they looked like a primitive tribal war clan still using melee weapons.