This mission is completely optional and is not required to finish the story. After finishing up Reuniting the Family, drive around as Michael until you get a phone call from Tracey. She says she is being stalked by someone and wants you to help her out, so choose "Yes" to start this mission.
Head to Tracey's location and get into her car, then go to the first location while looking for a purple convertible. When you get there, Tracey mentions that she doesn't see the car. Keep going to each location, until you finally spot the car at the fourth spot.
Now, you have the choice to either kill the stalker or scare him enough until Tracey says everything is fine. If you kill him, just shoot him while he's in the car. If you go the option to scare him, shoot his tires to stop his car, then just drive close to him until Tracey says it's enough. Once you're done, drive Tracey back to your house to finish the mission.