The trick is to keep it wet for a day or two

Of course the best prevention to avoid mold on paved walk and driveways is to add waterproofing to the cement when it's poured. That is seldom the case in Thailand. I have tried various methods in removing the ugly mess, including hazardous chemicals. I finally came across a non hazardous method which requires little elbow grease. Wet down the area to be treated.  Keep it wet for a day or two.  This might require that you cover it with plastic, paper or rags.  When it's thoroughly moistened, most comes off with a light brushing and a spray of water.  The nearly clear area took three minutes of scrubbing. The trick is to kill the mildew so it won't regrow. I used a mixture of vinegar and soda (yellow rectangle area).  Wet the area with vinegar , then sprinkled soda and watched it bubble for a minute. Of course to avoid a reoccurrence the cement surface needs to be waterproofed.