There is an aloe growing next to a street I walk along regularly that has rapidly become covered in what is known as White Aloe Scale (Duplachionaspis exalbidais) which is a species of armoured scale insect.
White Scale insects are immobile once they lock themselves into place to pierce the plant and begin feeding on sap for their nourishment. They do this by sucking the sap through a fine, thin feeding-tubes. From a distance it looks as if the leaves covered in what appears to be white fluff.
As you can tell from the first photograph above, if the infected plant is left untreated, all its leaves become covered with millions of scale.
What is interesting is that these creatures actually space themselves equidistant from one another on the leaf surface to ensure they have sufficient space to develop fully without being crowded out – although I think the earlier photographs suggest that their idea of crowding is very different from ours!