It looks like a fungal infection to me. You should dip them in Furan-2 made by API. Used for treating fungal infections in freshwater fish, but works great for zoas. One packet per cup of tankwater and dip all of your zoas for 15 minutes. They always come right back and it will kill all that crap on them. You can repeat the treatment after a couple days, but should wait a week after that.