In a society of conscious machines tasked with spreading the human race to new planets, a philosophical robot is sent to investigate the ghost-ship remnant of a previous (non-conscious) mission, but soon finds themselves facing a growing zombie-robot army that threatens both society and mission.

When an android sent to investigate the disappearance of a colonisation space ship discovers an armada of self replicating robots, he must stop the invasion before they reach earth.

I changed the zombie robot to self replicating robot, but body snatcher could also do the trick.
Also I made your protagonist an android as it sound better than philosophical robot or synthetic or AI.

I have fewer problems with the zombie aspect being overdone as I am confused as to what is a zombie robot.

Also, what do you mean by a non-conscious mission, are the passengers in cryogenic sleep? (In which you should just say that) or are the passenger's non-sentient robots? (In which you should just say that)

I guess my issue is, why would conscious machines do our bidding at all? As above, Zombie anything is a killer for my interest at the moment.

A conscious robot sent to Earth to use the planet as a resource, but doing so well end humanity not doing so ends the life of his race would be more interesting albeit similar to The Day The Earth Stood Still.

Maybe he stumbles upon the planet before we existed and most choose between the hours people at the end of their life of an untested people just at the beginning of theirs.

Having a robot protagonist is fine, even the idea of looking for a new home is good, but after that I feel it needs work.