It depends on a variety of factors.
1) are the aliens truly peaceful? A truly peaceful race has a lot better chance at being accepted than a warfaring race (though it should be stated that we would probably understand a warfaring race better, even if we wouldn't accept them per se).
But what if the peace is a fragile one, and one easily able to be broken due to simple misunderstandings? A classic Far Side cartoon has a race of aliens shaped like hands and the bumpkin who they first run into pick up their header and shake him, thus dooming earth to destruction.
2) what are the differences? are they simply made of different stuff than we, but generally configured the same with a head, 2 arms, 2 legs, and divided into different genders? (for instance a sentient race made of crystal aka diamondhead from Ben 10)
The weirder and more outlandish the differences, the more difficult of a time we will have understanding and accepting them. In the original Star Trek there's a creature called the Horta which is, by all appearances a un- sentient monster that's killing the miners, but Spock gets to the root of the problem by communicating with it to their mutual benefit.
3) what are their motives? In the Twilight Zone episode "To Serve Man" the aliens came and solved many (if not all) of mankind's problems, in order to use us as cattle.
4) Are we able to communicate with them? any sort of peace needs to be based on mutual understanding and communication. What happens if the form of communication is one that only other animals and not humans understand? What if said communication enrages the animals they come into contact with?