Simon finally feeling safe on the Hail Mary, finally letting himself believe that this isn't some kind of trick from the Eel, or the COI, or whatever God that may be watching. Finally believing that Grace really does want him safe, and happy, and alive.
Grace seeing this, finally feeling like Simon is stable enough to see the Greenhouse. I mean, the guy had a breakdown over the screen room, Grace was not going to risk his reaction to REAL PLANTS until he was sure Simon could handle it.
Grace excitedly leading Simon towards the Greenhouse. Opening the door. Watching for Simon's reaction, hopeful, but knowing it could be too much.
And it finally hits Simon.
Not relief. Not awe at the greenery, the abundance of life.
What else are plants meant to feed upon?