Aloe pests and diseases are difficult to combat unless you go on a vigorous campaign by spraying poisons regularly. 1.White scale is easy to identify as the leaves looks grey and on close inspection you will see little rows of white scale. I use 'Spray and Cook' to suffocate the scale. You could use cooking oil or Oleum or try dipping a toothbrush in 'meths' and scrape them off. Do get rid of it as soon as possible as they will destroy the plant.