I think you would be well advised to keep the zoas in a QT for a while before putting them in the main tank if you have found a spider......
Problem is, I'm not sure how long you should quarantine them for before its safe to put them in the main tank, in fact I was just about to post asking that very question, as I found 2 spiders on a small new colony of zoas and have had them in the QT for about 3 weeks now....