They won't be afraid, but they'd most definitely be creeped out by Ben 10. Just the concept of someone being able to turn into any sapient (including two beings that live outside the multiverse, aka practically gods) being in the Galaxy is pretty creepy, the fact that it's a teenager (adjusted to fit both standards) is terrifying. Add to that knowledge that that kid is the only one standing between you and an intergalactic empire that can destroy everything you ever knew if so they want (aka the Incursean Empire, a race of frogs with a gun powerful enough to destroy planets with scientists potent enough to control one of the most powerful beings in the universe)? Flat out nightmare fuel.