I have seen this before too. My advise is to cleean the underneeth of the glass but you should remove it first as the markers easily fall off if nudged. Glass removal of silverstones is normally pretty easy as the gasket tends go quite solid with age. Remove the movement/dail etc
then gently knock the glass from the underside (i use a tiny plactic hammer)or you can try pushing. The glass will go back in fairly easily as long as the gasket remained intact.
nic