If they are new, that one is probably still stressed. Sometimes they'll expel zooxanthellae when they are under stressed. Just leave it alone. It may take a few days or even a week or more, but it'll likely be okay.
Even if it doesn't make it, as long as the other one stays healthy you'll soon have plenty more to replace it.
Your rocks look pretty pristine from what I can see. Is this a new tank? If so, although zoas are commonly listed as good beginner corals, I had zero success keeping them until my tank was about 9 months old. Then all of a sudden they took off and now they grow like weeds. So if the tank is new, don't be disappointed if they don't make it. Wait a month or two and try again with a few heads of something cheap. At some point, they9 take off and then you can start adding more exotic ones.
