Aliens are a neutral party in Civilization: Beyond Earth. Don’t worry, they don’t actually have flying saucers and won’t abduct your citizens. They are however extremely territorial, and can become notoriously aggressive if your approach is relentless against them.
The real problem with aliens comes from their highly defensive behavior, which can turn into aggressive, tenacious offense if you lurk around their territory too often. You’ll already have some form of unwelcoming rival to deal with, so having the added burden of alien threat can really make managing your nation difficult.
How to Deal With Alien Threat
Aliens will attack Workers, Trade units, Colonists, and everything else that get too close to their nests, which can be easily spotted. What’s worse is that they tend to block off territory, and also stay dangerously close to miasma, which is already a hazardous nuisance that you will have to deal with.
How you deal with aliens should be something you decide right from the start of the game, as running into them is almost inevitable. The safest and best way to deal with aliens is to avoid them at all costs. Simply keep yourself aloof of the alien tiles and work your way around them. Use ultrasonic fences to keep them away from your cities.
This may not always be the most efficient method however, as often alien nests tend to block your path towards valuable resource tiles that you could capitalize on. Additionally, the ease of expansion and growth of your city is lowered.
Then there is the more aggressive method, which means clearing off alien nests. This takes up much more Energy and Production from your end, but it also brings some benefits. You get good Experience from fighting aliens, and also tend to get valuable resources from their nests.
Aliens aren’t pushovers though, so you’ll need to train a strong military force early in the game to deal with them. That itself can be beneficial, as you will have a larger and powerful defense than your rivals.
However, if you have an aggressive approach towards the aliens, they will begin to get more agitated towards humans. This is evident by the color of their icon (green is neutral, orange is hostile, and red is infuriated).
If the aliens become infuriated, they will begin attacking human cities (yours and rivals’) and barging into your territory. Aliens have quite a few units coming out of their nests, though their most powerful one is in the ocean. Be wary of this if you have any coastal cities.
The longest yet best approach is actually to befriend the aliens. This takes quite a bit of time, but the rewards outweigh the number of turns consumed. To befriend aliens, you need to buy a tile that has an alien nest, then refrain from attacking it for 60 turns. After that, the alien icon turns blue, indicating that you are now friends with them.
Befriending aliens will give you a powerful ally that can be used for defensive and offensive purposes. You can move freely through all alien tiles from there onwards, which gives you massive strategic advantage over your rivals.
It is nearly impossible to remove aliens from the game without conquering the entire world, but their influence generally starts to fade as your nation (and your rival nations) expand. New alien nests only spawn in neutral territories or in unimproved Xenomass tiles.
At the same time, humanity gets better equipment with time to deal with miasma and alien threat.
Aliens and Diplomacy
Your approach towards alien nests also has an effect on your diplomatic relations with other nations. Some nations will have an aggressive approach towards aliens, while other will be more apathetic.
If you are constantly attacking aliens, many colonies will condemn your actions and may turn against you (even start an all-out war). Similarly, you could anger a rival nation if you befriend aliens, as they may have a ruthless approach towards these creatures.
You can’t keep everyone happy, so it is always best to do what would benefit your own colony the most. If you good relations with a powerful nation, you might want to compromise, but otherwise you should always prioritize your own nation’s prosperity when it comes to dealing with alien life.