My thoughts on this is that Greta is not malicious. My theory on this was that the captain had been there in her hive for some time. After many different simulations, Greta had been conditioning the captain to eventually reveal her true form. She's been through this before with other travellers that have suffered the same fate and has adopted the "simulation" method because to her its proven the most effect way to treat those she encounters. This is probably for her own self protection as many creatures put in the captain's position are prone to react violently (this is seen when he slams her against the glass).
However, the captain's subconsciousness has been fighting back against the simulation. To me the female crewmember that freaked out was his subconscious rejection of the simulation. After that happened, Greta knew she was out of time to condition him and revealed the truth simply because she had no choice.
After the captain saw the truth he was so mentally broken it seemed to me that Greta was able to put him back in the simulation with less resistance. He expression at the end and expression throughout the simulation indicate to me that she was truly sad that she needed to expose him to that level of mental trauma.
Note that I don't think she planned on feeding on any of them because the corpses of the other two crew members were shown and they:
A.) Looked to be dead for a long time
B.) Were not devoured
C.) Didn't look like they were preserved for the purpose of being used as hatcheries, they were clearly dead and dried up.
I do think that captain is dying and that Greta probably does not have the means to preserve him. To me she is just trying to bring him comfort as he passes on.