White scale can be a pest on aloes and are often difficult to eradicate completely.  The best thing to do is to spray the plants with chlorpyrifos or a mixture of mercaptothion and half-strength mineral oil.  Mix the mercaptothion with water according to the manufacturer’s instructions, and mix with the mineral oil at a ratio of 2 tablespoons of mineral oil per 5 liters of the mercaptothion-water mixture.Follow this up with a second application 10 days later.  This should clear up the scale but is may be necessary to treat the plants once or twice more if the scale is very persistent.