Having had experience (though not with guppys) I can tell you the white fuzz on her tail is probably fin rot. You'd be doing yourself (as well as the fish!) a huge favour to isolate her and treat her with Maroxy *right now* (it spreads quickly and will eat her whole tail fin), maybe adding a drop per 2 gallons of tea tree oil to her QT tank (or add Melafix, but don't go out and buy it! It's just watered down tea tree oil!!) The oil will help her heal.
Other than that, I'd keep and eye on the one with the moving/disappearing gravid spot. I've never taken care of guppies, but I doubt if that's a good thing to be seeing in any fish. Maybe some one else has some answers!
Best of luck!
Jade