The use of 'zoa' typically implies one of the many types of zoas (Zoanthus, Palythoa, Parazoanthus, Acrozoanthus, etc.), so it is a generic term. Zoanthid can also be a generic term because it is not describing a specific genus (as Zoanthid is not a genus).
I pronounce Zoanthus as Zo (long O)-anthus, but typically pronounce 'zoa' as Zoo (like a place where animals are held in captivity

). Pronounciation is a tough one to hold down... There is a Versailles, Kentucky that is pronounced Ver-sails instead of Ver-sI. Cairo, Indiana (I think it's Indiana, anyway-could be Illinois) is pronounced KA-ro (long A) instead of KI-ro, so cultural differences (whether the be correct or not) are often the decider (of that's a word).