White scale starts with what looks like a coin sized spot of white dust. It will gradually cover more and more of the leaf upper and lower surface. Eventually this deprives the plant of sunlight to the extent that it can deteriorate and die.

They are stationary insects that are easy to kill, but they tend to remain where they are, creating the impression that the insecticide has had no effect on them. Use any contact insecticide to spray with, or if you prefer not to use poisons, a soapy solution of green  laundry cake soap (real soap) will also do the trick, provided it is not washed off by rain. After 10 days or so the scale should be removed with a soft brush so that any new population can be spotted straight away.

Some people prefer to brush off the living insects (seeing that they have to be removed after spraying anyway). After this a preventative spraying with an insecticide is optional. White scale can also be removed, from places that are difficult to reach (like a tall Aloe Barberae) with a strong jet of water.