Obviously the temp on Mars is much different than Earth. Extreme cold at night time. I'm assuming they are bringing a ton of solar panels for energy.

Air most mostly carbon monoxide on Mars. So any cracks or leaks on the ship will kill everybody within an hour. I'm sure they have plantlife with them and they are going to try and grow some.

Alien spores and fungus. Please see the video game The Last Of US.

There is water on mars but it is frozen. Doesnt mean they can just drill into it and drink it up but if they need to replenish their supply that is an option.