The Celestial Slug simply floats toward you for as long as you're on the planet. It cannot be damaged, as attacks just phase through it. "Hitting" it, though, drops a single Magicite for each player, up to four per slug. After that it continues to follow the nearest player slowly, doing 2 damage upon contact, and still unable to be physically hurt. It spawns 100% of the time, as long as the quest for it is active. If nobody has the quest for it, it will not spawn. You need the Magicite it drops for Captain Atlas's third quest in the Galactic Fleet storyline.