In the following section of the guide to Marvel's Spider-Man, you will find a breakdown of the "Tick-tock" side quest.
Mission's starting location: Greenwich
Reward: 1500 XP.
Talk to Philip Chang. He will ask you to find another student - Ethan. This time, the hint is a picture of a rail crossing, which suggests the Harlem district. You have to go to the place shown on the screen above.
Talk with the victim. Then go to marked location. You'll see the man you're looking for being chased by a group of bad guys. Join the fight - your opponents will be ordinary people who have been transformed into zombies. After the fight, approach the closed door, open it and save Ethan. The side mission is complete - you will unlock the next side task.