Calcium binds the proteins into a network that resists dissolution in aqueous solutions. If the calcium content of the water/whey is too low calcium will be dissolved out of the cheese and the proteins become unbound and slimy. If this happens remove the cheese, wash the slime off under running water and replace the cheese in a salt solution with the proper pH and the proper amount of calcium. Caldwell states for feta aging brines to use 0.74 pounds of salt, one teaspoon of vinegar and 1 tablespoons of liquid calcium chloride solution per gallon of water. I found this to be too low in calcium so doubled the calcium and had no more slime. The slime is not coming from bacterial decay.