Can Eridians even get "scared" by surprise? Humans scare each other all the time if they come into someone's sightline very suddenly & quietly. Doesn't even have to be that quiet sometimes, if we're totally engrossed in something else we aren't as aware of the sounds around us and can get surprised that way.
I suppose it could be possible for Eridians to get really focused and maybe get surprised if there's some sudden sound/movement, right? But it's not like they would open a door and jump because there was another Eridian on the other side about to open it themselves, they'd hear through it, like Rocky hearing where Grace is through walls in the Hail Mary.
Anyways all this to say the first time Grace comes around a corner not expecting Rocky in his ball right there at his feet I bet Grace jumped, shrieked even, & then had to explain what the heck just happened lol. Or Grace is doing some science in the lab super focused on what he's looking at and Rocky brushes his leg while passing by and he falls right off the stool
Rocky would be delighted by the discovery, and start taking full advantage of this human weakness, at least while Grace is doing well at the beginning of their journey to Erid.