Evil

I can't remember where I first saw this information, but apparently Rocky's aim was to make the wall to the tunnel out of many different materials to find out which one Grace is able to see through easiest, not based on opacity - like a human or something that evolved to see light does - but based on acoustic impedance. For anyone who doesn't know what that is, it's a way of measuring how easily sound passes through a material - usually used in ultrasound scans. Therefore, for a species who uses echo-location, he's working out which material is easiest for Grace to see through - and from what I can tell he just got lucky with the clear Xenonite. I'm also fairly sure that he was completely unaware about species that see through light waves rather than echo-location, based on his reactions to Grace in the movie.

Basically, Rocky wanted to make things as accessible as he could for any alien that he could come across with the limited knowledge that he had of how different species see things. He is essentially setting himself up to be as accessible as he can for anyone he could possible meet out there. Whether its due to desperation for connection, or just because he's Like That, that's why I think he's so neat. His first interaction with Grace is just him trying to make Grace as comfortable as possible.

I don’t care whether he’s just smiling or talking with Grace or being brutally atomized or something just give him to me