Answer: Prunicola scale usually infests Prunus plants such as cherries and plums as well as lilac and privet so I am not certain you would be seeing that type of particular scale on a crabapple (Malus).
However, crabapples are occasionally subject to scale infestation (as well as a variety of other problems.) Scale insects are best treated with a dormant oil spray when the plant is dormant, or with a spray such as insecticidal soap targeted at the time the crawlers are active. (Spraying at the wrong time will not be effective.)
I would strongly suggest you consult with your local county extension to obtain a specific identification of the pest and based on knowing that, decide how to proceed. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to apply it for maximum results. Unfortunately, without any geographic or state location information I can't give you contact information -- but you could check the following web site.
Good luck with your tree!
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