Extreme speciesism, people Claiming men are "the race of god" (even though really it being the other guys is just as likely)
There will be people of course lobbying for peaceful relations, these groups would mostly be made of the racial minorities. They will of course slowly, but eventually get the extraterrestrials to have equal rights to men, but the speciesism doesn't end there.
All the while people of both worlds are beginning to understand and communicate with those of the opposite species. The differences people find will only serve to widen the chasm of xenophobia.
immigration between planets, both ways begins, this only slightly improves relations
However, here's where it gets better, it'll be humanity's natural greed that saves us, A brilliant CEO puts his prejudices aside and realizes, "I can make a metric fuckton (totally a real unit of measurement) of money off of this".
Quickly people of both sides realize that there is an entire new market for pretty much anything, there is still some prejudice from both species.
Then, a new generation is born, From birth they know that humans aren't the only people in the universe, the parents who are still afraid of the extraterrestrials will in turn try to instill that fear into their children, this will fail for one reason, history class, kids will realize that this is the same as the blind prejudices of the past. When this new generation has many of it's members reach adulthood interplanetary trade will flourish leading to a galactic golden age of industry.
And this isn't the best case scenario.