Scum dots indicate highlight dots that are too small to transfer ink properly for a given print condition.
If full AM is being used, you can increase the minimum exposure value by increasing the amount of the bump curve. For a full explanation, see Determining UV exposure times and minimum exposure values. If full AM with a bump curve is being used, and scum dot appears at the edge of a gradient, check that the minimum dot size has been set in the output process template to clear out all dots below the bump curve value.
Additionally, if HyperFlex is not currently being used with full AM screening, try using as much HyperFlex Advanced as possible at the highlight minimum dot. For full instructions, see Implementing HyperFlex with Maxtone.
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