@A-tanI can get behind Phos's plan because at least he is taken direct action to exterminate the moon plague. Everyone else (including the gems that went with him to the moon) grew complacient.
I get they are both being used, and maybe that's exactly why Phos is telling this to Cinnabar, because he knows the moon bastards are using him and hiding information from him, but also he just doesn't care anymore. He got to the breaking point already.
I’ve read the manga up to vol 6 a few times and to see the characters I love get to this point makes me so uncomfortable