I’m not sure why an advanced alien civilization would travel inside a giant rock on a course toward us. You’d expect a superior race to use something a bit more sophisticated.
That “rock” might be equivalent to barnacles on a sea going ship; debris picked up traveling through Worm Holes and such by way of electro static charge. Similar to powder coating metals for automotive and industrial parts.
And maybe that rock is a special delivery gift from none other than Klendathu meant to “Rock” our world!