Black Floor Tile Grout Has White Spots?

My ceramic tile floor is over 20 years old and in great shape. The problem is with the grout, but only in certain places. It looks like someone poured milk on the black grout and let it dry. It has white marks, but only in certain places. I have washed the grout with everything I can think of, Mr. Clean, white vinegar, and hydrogen peroxide. It looks great when wet, but when it dries, the white flaky look returns. Has anyone had this problem and what is the solution?

By Cisco's Mom