This happens when the sauce begins to split. The sauce can split for a number of reasons including having too much fat and not enough starch in your sauce. The cooked potato in this recipe helps the fat from the cheese and butter to emulsify into a super smooth sauce. Also, overcooking the cheese sauce. When cheese hits a high temperature, it curdles and creates a grainy sauce that sticks to the pan. Always cook the cheese sauce over low heat and use a spatula/spoon to prevent it from burning.