Micah-10 knows that the war against The Witness and his followers in Destiny 2 will prove costly for The Guardians, which is why they absolutely need reinforcements and want you to find other Ghosts during The Return quest.
Destiny 2: Dual Destiny Mission Walkthrough Guide (How to Get Exotic Class Item)
For The Return quest, Micah-10 wants you to track down a Ghost. To track down this Ghost, you'll need to find him during a certain raid. You can choose to start this raid whenever you're ready. Reinforcements are always good in Destiny 2, but finding these Ghosts can take some time.
To track down this Ghost, head to the Cosmodrome and complete The Disgraced strike. You can launch this strike from the Cosmodrome or complete it randomly via the Vanguard playlist in Destiny 2. For anyone who only wants to focus on completing The Disgraced strike, launching it directly is a much better approach.
It's a different approach to the missions Micah-10 has already given you, such as having you work on Convalescence Greenery or the Underbrush missions.