I've been rereading this manga since I'm trying to archive the pages and the fact that Sensei 'recognizes' this Lunarian reminds me of something that happens in future chapters. I don't remember exactly what happens, but the jist of it is is that the Lunarians deploy this... thing that Sensei can't destroy - not because he couldn't but because he won't. I believe it was implied that the thing they deployed resembled the person who made Sensei, hence why he won't destroy it. I don't know where I'm going with this, but what I think I'm trying to get at is that perhaps the Lunarians were trying to do something similar here?
By showing Sensei things that he was familiar with when humans were still around (the person who made him, a dog he/his creator had) maybe that will get him to pray?