I could imagine that something like the slime mold could be an alien.

Single-celled amoebae can remember, make decisions and anticipate change, urging scientists to rethink intelligent behavior.

They are one of the oldest organism on planet (almost a billion years old) they have no brain but are able to plan, anticipate, remember and even share information. We can find them a bit everywhere on the planet and yet barely anyone knows about them.

If you ever encounter one they love oatmeal but hate salt.