If it were scale, it would be raised above the surface of the paster and feel rough. You'd have to dive down with a mask on to really see it clearly. You could try wet sanding a very small area with a fine sanding sponge to see if the variation goes away (you remove something).
Since this is a dark plaster color, it honestly looks like some mottling that occurs do to poor workmanship OR it's just color bleaching. Many people avoid dark plasters because, if they can survive the initial application without getting bad mottling, they typically suffer from color bleaching over time. Even the blue colored plaster creme in my standard Caribbean Blue PebbleTek (probably one of their most basic colors) has faded noticeably over the years. I can only imagine how quickly darker shades of plaster degrade over time.
