How long of a piece of work is this? I have a situation very similar to this in the novel I am writing, but I'm not keeping that from the reader. In fact, the reader should be amused by the mean old bad guys pretending to be nice. It is even critical to the plot because one of the bad guys has a change of heart after living with the good guys for a year.

If you want the reader to believe the bad guys are good guys, you absolutely MUST stay in the human point of view. The reason is that once you move to a point of view that knows the details, you MUST tell the reader those details. If you don't, the reader will feel cheated and probably not read anything else you write. That is important, because only JK Rowling, Stephen King, and a few others can live off the income of only one of their books, and to get there, they had to build a loyal fan base by writing many books.

And remember, on a novel, the basic plot will be on the back cover and perhaps even the front cover. There will likely be a picture you don't like, that doesn't look anything like your aliens gracing the front cover. So the readers will probably know about your aliens, whether you tell them or not.

My suggestion is that you either make it clear that the aliens are pretending so the reader can watch the humans figure out the plot, otherwise stay very close to the humans and keep the alien secret from the humans so you don't have to tell the reader. But, as soon as the humans learn the truth, so does the reader.