If you look at the possibility of space travel in a frame of reference of what we are currently experiencing on Earth, I think you'll agree that it is very unlikely that we will be visited by any in alien in the future (or the past but I won't go into the temporal dynamics here).
The amount of energy required to produce the propulsion needed to do the light years to get to here, would absolutely blat all their resources.
If they had enough resources in the first place, then they wouldn't need to come here.
Also, if there was more than one of them, they wouldn't need friends, and they wouldn't come here for that reason either. And a single alien wouldn't be able to do it (unless it was the Alien at the beginning of the film, er, Alien).
Also, if they needed us for food, then the packed lunch they would require would have made the whole expedition a chicken and egg situation