Many of the visiting alien people can change the shape of their bodies.
It's just a natural ability that some races have and it doesn't mean anything negative about them.
Even when the alien people can shift their shape, that does not change their ability to live here.
Many races can change their shape into anything.
But they still can't be us and live here. 
All people look like people.
Some races have more than one shape though, because they can change at will.
And some do not have an actual standard shape at all, but they still prefer the people shape.
Alien animals look like animals, and alien plants look like plants too.