Why assume they are harmless?
Not saying they are harmless, it is always assumed that they are hostile. Stephen Hawkings posited that advanced races would most likely be machine based AI. By surpassing the abilities of their creators, conflict would be inevitable. The AI would seek to eliminate inferior biologics. That is the most plausible explanation for a potential hostile alien. The other side is that life is extremely rare and advanced civilizations would protect and nurture our development.
My take is that being alien, their thought processes might be diametrically opposed to ours. Example: they enslave us all, but we only have to work 3 minutes a day, pulling weeds. In return, they provide us with unlimited energy and food.
Example 2: they threaten to destroy us, unless we all shave our heads and shout "KAHLOO-KALAH!" at the sun each day at noon.
We simply don't know. Some say octopuses are aliens. Others say tardigrades are.
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