Maybe a q tip will do the trick?In a professional vein watch repairmen use a barely abrasive compound on watch crystals.
Or else you might try auto polishing compound., the grade used to create the mirror finish. (Avoid the harsher rubbing compound.)
Telescope makers use various grades of carborundum, and finally a rouge compound.
It's natural to rub with our fingertip. However there ought to be a narrow tool that rubs compound only in the scratch, yet not abrade neighboring smooth glass. I've never figured out how to do it.