mushrooms will attach where ever they are happy. Personally I wouldn't be too concerned about it unless those zoas are real expensive. Just a thought, don't know if it would work, but maybe try moving them into a highflow area and see if they'll detach from the zoas...They like flow, but they don't like to be beat about the head. Beat them about the head a little.
Others may have some better ideas for removing shrooms form zoas...