The only thing I can think of is some sort of bacterial bloom or a protien scum which are often white or whiteish. These are caused by an excess of nutrients in the water. I would recommend cleaning out as much as you can and then peform a large 50% water change twice a week for a couple of weeks to lessen the concentration of whatever unknown nutrient might be feeding it. It wasn't there before so we presume it's not in your tap water.