You mean some animals in Europa (the moon of Jupiter) ?
Or in some spaceship hidden behind the Moon?
If they have spaceship then they are very advanced. Why dont they just live amongst us?
There could be aliens on pretty much any planets in the solar system, so why not? We only see good pictures of them that get revealed by Nasa or some other agency. Even if they had a picture/footage of aliens or spaceships I'm pretty sure that it is going to be classified.
If they look like us, some might live among us. It really depends on how you define an alien; someone who is not from Earth?
I`m sure that there are no humanoid aliens in the solar system. There might be some fish or bacteria or algae in Europa's ocean, probably more deep sea creatures since its a very cold ocean.
If there are aliens, then its 100% that they are not from the solar system. An alien's definition is a being not from earth.