Planets capable of supporting life each tend to have potentially different biochemical conditions and the tendency is for species to evolve to adapt best to their particular planet’s characteristics. Aliens will thus find Earth to be at least somewhat hostile as a habitat without substantial terraforming. This could explain why aliens don’t invade us even if they know about our planet. The uniqueness of each life ecosystem means that a given intelligent species is far better adapted to life on their home planet than anywhere else, and the odds of finding another planet exactly like one’s own is exceedingly low.
In particular, eating xenobiological matter for nutrients runs into the issue that, at least for humans, we require a particular assortment of amino acids, and an alien biosphere is likely to only have some and not all of them. Thus, while we could eat alien food, it may not be complete nutrition wise, much as a vegan diet runs into the problem of B12 deficiency. While it’s possible alien biologies could evolve to be able to have complete nutrition from any organic matter, the fact that this didn’t happen with humans suggests that it may not be common, and could lead to dependencies of native biology.
Thus, most likely the nearest alien civilization will not find our world hospitable for colonization. Perhaps further away, other more similar species can evolve, but they likely would have to traverse through territory occupied by others, which may not be reasonable in interplanetary diplomatic terms.
Of course, sliders from a parallel Earth who share our biology would probably find our world desirable. However, the local time travellers of this universe are likely to resist incursions by sliders for obvious reasons. Advanced enough time travellers would be expected to have similar levels of technology as sliders, so the advantage of local knowledge probably makes invading other parallel universes very difficult.
There’s also the small probability of panspermia occurring, such that two nearby habitable planets share the same basic DNA biology system, but the odds of this happening seem very low.
This also could explain what kinds of advanced aliens to expect to be present around Earth. Most likely, the Earth is valuable to them not as a potential colony, at least not more so than any other arbitrary planet with low level lifeforms. Rather, the unique biology is a goldmine for scientific research into understanding what biologies are actually possible, and so Earth’s main value to them might be as a preserve of diversity. Thus, they’re unlikely to care much about our local politics, and would only intervene to make sure we don’t nuke ourselves, or otherwise destroy the specimen of life that is Earth.
The most imminent danger of an alien biology is simply the existence of xenobacteria. While some microorganisms like viruses depend on particular biologies and therefore alien biologies are effectively immune to them, other microorganisms like bacteria can be harmful simply by consuming matter and reproducing normally, as well as potentially releasing toxins inadvertently.
Immune systems are generally adapted towards targeting familiar threats, and as seen with the way deep sea bacteria are effectively invisible to mammalian immune systems, are not effective against totally alien microorganisms.
https://www.livescience.com/deep-sea-bacteria-invisible-to-human-immune-system.html
Meanwhile, such microorganisms are very capable of still causing disease.
https://www.sciencedirect.com/science/article/pii/S2090123222001230
Thus, any significant exposure to an alien biology carries substantial health risks. Particularly, entering an entire ecosystem via something like a colonization or invasion effort could expose one to many potential pathogens. Generally such efforts would have to be preceded by considerable study and research to determine the ways in which such pathogens can be mitigated.