I think the cloudiness in a wit comes from the yeast in suspention. Irish moss helps to clear beers by precipitating more protein after the boil and won't do anything fot yeast. However, since you're aiming for a cloudy beer because of the style, i wouldn't bother using irish moss.
Another thing to consider, adjuncts like wheat will contribute more to chill haze which may be relevant to add to the cloudiness from the yeast. However, i'm not expert w/r/t belgian wit. Somebody else should comment on that.