I looked on a solubility chart to answer this question, and hydroxides are generally insoluble (with some exceptions of course). However, it says to consider [tex] Mg(OH)_{2} [/tex] as an insoluble substance, though it may be moderately soluble.
The answer that you are most likely looking for is: Yes, a precipitate does form - this is due to the double placement reaction:
[tex] Mg(NO_{3})_{2}_{(aq)} [/tex] + [tex] 2NaOH_{(aq)} [/tex] → [tex] Mg(OH)_{2} {(s)} [/tex] + [tex] 2NaNO_{3}_{(aq)} [/tex]
