I don't know if the laying down was intentional or not, so forgive me if I'm telling you something obvious, but whisky should always be stored upright. Some advocate turning it sideways occasionally to wet the cork, but it should never be stored that way.
Various weird sediments turn up now and then--some are likened to dandruff. Is your white spot like that, drifting as you turn the bottle, or is it affixed to a particular point? In any case, it's probably nothing to worry about. If it upsets you, invite us all over and we'll give you our considered opinion as to whether the flavor is spoiled or not!