As I've posted elsewhere...
Why would aliens come here in the first place? The most abundant elements on Earth are the most abundant anywhere (Hydrogen, Carbon, Oxygen, Aluminum, Nitrogen), so there's nothing special physically. I doubt they'd come to stay considering how lethal the place is. Even assuming they can cope with the temperature and air pressures, they couldn't live here without serious engineering. I'm from here, my family has been here for 4 billion years, and yet 70% of the planet is lethal to me if I'm exposed to it for more than a few moments and that's before we look at places that are too hot, too cold, too dry, too rocky, etc. Just about everything alive can kill us, from bacteria to viruses to fungi to animals. Even the planet itself goes out of its way to kill us. Earthquakes, tornados, volcanos, mudslides, blizzards, etc. Physically any alien would be better off building on a lifeless and geologically dead body like the moon.
But even if all that doesn't put off the aliens, what about the dominant species? There's this hairless ape with an appetite for violence that has made entertainment out of killing each other off. They claim they don't want to kill each other, but do it at the drop of a hat and over reasons like which mythological being is the right mythological being or ownership of shiny rocks. Given how much these apes delight in killing each other, is there any doubt they won't do the same to an alien traveler?
Any alien species with the technological ability to go through interstellar space should have the technology to do whatever they need without coming to Earth. Need iron? They can mine an asteroid belt. Have DNA problems? We're already engaging in genetic engineering and even if they had some genetic flaws, there is no reason to think our DNA is anywhere close to compatible with their without tons of engineering, so why come here in the first place? Food sources? Think of how many plants and animals are poisonous to eat. It seems unlikely aliens can digest a pork chop or piece of bread. Onions can kill my dog; why would it be reasonable for an alien to be exempt from those problems?