Pretty straightforward. If you have alien walls in your inventory, hover over a placed alien wall, and press Q (the default pipette tool), then it will not put the wall in your hand, but rather error and (based on my settings), changes to ghost alien wall.

Yeah, so this is where the mod gets interesting. There's an item for the walls and gates, but there's no corresponding entity that get's placed and vice-versa. So when you go to press Q, the game looks at what you've moused over (HybridWall-X) and looks in your inventory for it. But it doesn't exist, so it beeps at you. It's also why bots won't place the walls from a blueprint.
I think that, in one of the recent updates, I might be able to do something about that, but I'm not 100% sure.

This has been fixed.