i apologize if i misread your post. but if it were a build up of water conditioner enough to cause the condition that the op is describing would most certainly be lethal to any living animal within any tank that the sprayer is being used on. especially since that much build up would be highly concentrated.

most likely, the muck and slime is an algae build up within the bottle. possibly, some anaerobic bacteria as well. there also could be underlying factors involved that the op has not mentioned, i.e. tap water quality,.