The first differential diagnosis has to be struma ovarii, which is a monodermal ovarian teratoma that is usually cystic. Struma ovarii may be associated with mucinous or serous cystic neoplasm of ovary, so finding struma ovarii is not enough. Some of the lining epithelia appear mucinous, but I cannot be sure without high power views. All epithelial lining appears benign cytologically. One has to examine the entire ovary carefully for other neoplastic or teratomatous components.