The suffix -etto (-etta for the feminine) is often used in Italian, appending it to a noun, to denote a smaller, and often nicer and cuter, version of the object or creature denoted by that noun. It's called a diminutivo or a vezzeggiativo of the original noun.
So, a casetta is a small, possibly nice, house (casa); a pescetto is a small pesce, if Chiara is a child, or a close friend of yours, you might possibly call her Chiaretta, and so on. Note that this process is not automatic: not all nouns admit this suffix (for instance, a small cat, gatto, is never called a *gattetto, possibly for euphonic reasons).
Dictionaries tend not to register regular derivations of this kind (or other ones that are used similarly: -ino, -one and so on) or to just list them quickly at the end of the main lemma if they are often used in that form (as is the case for mossetta under mossa), unless the new word has actually an autonomous meaning. For instance, libretto, while literally meaning “small book” (and being occasionally used in this sense), has as its main meaning that of a “libretto” (English), the text of an opera.
So, in our case, a mossetta is simply a small, cute, perhaps simpering move. How to exactly translate it into English is beyond the scope of Italian.SE.
For more (in Italian) about this phenomenon in the Italian language, see the articles “alterazione”, “vezzeggiativi” and “diminutivo” in Treccani's Enciclopedia dell'italiano.