That rock has some serious film on it, cyano, and the possibility of pests can’t be ruled out.
The entire rock should be dipped if the zoa can’t be removed. And it’s probably better this way because you can kill all the films and algaes on the rock itself instead of just the zoas
We can treat this for algaes, films, bacteria, pests, etc all with 3% hydrogen peroxide.
You will need a container to dip the rock in and a batch of sw to fill the container.
Something like this would work well
https://a.co/d/82nmIyI
Then you make a batch of sw to put in it, and add hydrogen peroxide and stir it up
The ratio is 1:3 or roughly 30% hydrogen peroxide to 70% sw.
350ml per liter
Remove the rock and inspect; remove any crabs, hermits, etc
Place in the dip for 3 minutes. While in the dip, baste it all over and blow off those slimes and films and try and blast the dip inbetween all the zoa polyps in case pests are hiding. Get all the nooks and crannies of the rock. After basting for 3 min, let it sit in the dip 1-2 min longer. Do a quick baste to clean it off and put back in the tank.
In 24-48 hours that rock should be looking much better and the zoanthids hopefully will open again now that they are cleaned.
Hydrogen peroxide is an antibiotic and this will kill all bacterias on the rock and colony - good and bad. This is ok, because you have an established tank and there’s plenty of bacteria they will naturally repopulate it over the next few days and weeks. This will also kill any algae on the rock and pests too