Spider web should come off easily, u sure it's not fungus
As for cleaning I like things to be quite moist, not dry-windex is good as its streak free, if it's fungus I would be hitting it with acetone (though be careful with this as you don't want it getting to the edge of the corrector pate, be careful to keep it on the glass
I do have a box of zeiss cleaning wipes on the way but they won't be here till next week, I bought 200 so can spare you one