I believe humanity will (mostly) be able to cooperate with the ugly aliens
Things to keep in mind:
We do not know their intentions, or if we can really trust them when
they say they want peace.
They do not know that earth is not united under one government, or who our leaders are.
We do not know the full extent of their technological capabilities.
Here is how it would likely play out:
The aliens are unable to communicate with us via technology, as our software can't open attachments with the .alien file extension.
So they make a landing, and visit us in a city. Cities are full of people, there must be somebody that wants to communicate.
Landing in their spaceship causes a commotion, and their identity as aliens is confirmed in the minds of the people. We bring in linguistic experts and mathematicians, under heavy protection, and begin communicating with the aliens. We are not sure if they will destroy us if we offend them, so we are extremely polite in our communications, despite their disgusting appearance.
We warn them that not all humans are friendly, and 'Alien Safe Zones' are set up where populations are educated about the aliens and the government over that area guarantees the safety of the aliens. Eventually, curiosity overcomes revulsion and the safe zones become hubs of information exchange. The aliens soon realize how hideous we perceive them to be; they are hurt, but their desire to collaborate is stronger than their pride and they start to wear 'clothing' to cover up their body parts that we find most unpleasant. Despite the aliens and our governments best efforts, some humans still want to remove the aliens from our planet by force. Fortunately, due to the aliens friendliness and the
technological advances that they brought us, these humans are a minority. The aliens eventually learn about and greatly admire the human Martin Luther King Jr, and adopt his nonviolent methods to bring about change.
Over a few generations, we learn to live with each other and eventually visit them on their home world.