Dead or dieing zoas just look closed up, they will slowly shrink and then just disappear. I've had some odd weird ones that just die for no reason when the rest of my polyps are all happy, maybe I didn't acclimate them well enough, who knows.
I've heard of sexy shrimp eating zoas if they are starving but if they are kept well fed they should be fine. I don't know if it's the same for peppermints, I'd assume so since they will eat aiptasias as well. Just make sure your peppermint gets some food and you should be fine.