Mark an Alien Egg in Fortnite is a Week 7 challenge, giving you a nice bit of XP to level up your Battle Pass while you bang out this week's challenges.
Mark Alien Egg Fortnite: How to Complete Week 7 Challenge
Alien Eggs are present in many of the new alien-themed locations added since the Alien Invasion event.
In many of these locations lies the Alien Egg, pictured above if you are having trouble spotting what they look like. If you have trouble finding a specific location of an Alien Egg, your map will have dotted objective markers to help you pinpoint the eggs more effectively.
The very center of the map and Holly Hatchery are just two of the many locations where the Alien Eggs are present. Once you find one, simply walk up to the Alien Egg and interact with it, marking it and completing the Week 7 challenge.