They could have any planet in the solar system except this one. All of the rest of them are useless to us anyway. Despite what Elon may think, colonizing Mars is not only going to be extremely, unimaginably difficult, but it's nearly pointless. Establishing a Mars colony would be an ego boosting process, but nothing more, and it'll never be self-sustaining.
There are a few moons in the solar system that may be of interest to us in the future, but the planets are all useless to us.
If we're looking to establish permanent, off-world colonies, then we should be looking at some sort of O'Neill cylinder-type habitats, or maybe some of the larger asteroids in the belt. It would actually be much easier to establish self-sustaining colonies in space itself than on a planet like Mars, or even Venus. At least you don't have to fight gravity and the weather to take off and land that way.
If the aliens think that they can do something useful with Mars, they can have it, because we can't.