Generally speaking, I would say that this Marta does not see Mary Stuart in an objective or clear way. Marta's perception of Mary Stuart is being altered by this "haze" (foggy, unclear, murky) of "white veils" - which may be a reference to Mary Stuart's habit of wearing white veils (I think she even wore white when in mourning for her husband). Without more context though, it is hard to say for certain.