It is water and it happens even with some blizzlike (pre)WotLK map as well. Like schlumpf said, hit F4 to hide water and that shall be OK. I don't know how to really get rid of it, but you may try to just save those ADTs in Noggit and see if that will help you. If not, you still may try to actually re-make water on those ADTs and at least that should work. If not or if you can't re-make water (in case there are rivers with angled water, for example), then I don't have any other advice for you.