The best way to get rid of the white residue is to scrub those areas with a cloth or q-tip as part of your IPA bath finishing, before you do any UV post-curing.
Exactly why it forms is a bit mysterious, it seem to vary depending on resin type and the shape of the feature. My best guess currently is that it is partially cured resin that mixes with water or alcohol (becoming slightly dilute) and the cures, forming a weak powdery solid.
I have had some success reducing it when there are areas that I cannot reach mechanically by some combination of increasing the exposure time during printing, and by increase the duration and agitation of the IPA bath.
MakerJuice and FTD seem to be more prone to forming this type of residue, whereas 3DM-ABS rarely does.