This is great shot of Cottony Cushion Scale on a citrus leaf which was brought into the nursery for identification. It shows them sitting along a citrus leaf, along the middle rib. This is where you will normally see them congregating.
Unfortunately spraying is not really an option, as any spray that will kill the scale is not selective and you will end up knocking out your garden’s ‘good guy’ predators. This could also compound the problem by making future infestations worse. Your best friends in this situation are two of your garden’s natural predators… ladybirds and green lacewing. You can boost your garden’s natural pest-fighting ability by releasing some of these natural predators yourself.