What is Alby afraid of and why?

In the maze runner? He is afraid the the grievers, and he saw a little bit of what happen before the maze and what they were doing, the doors wont close, everyone is dying and the freaking greenie is crazy (both of them Thomas and Teresa)

Alby's memories include the Flare, of which he is terrified. His memories include the terror that accompanies the sickness, and Alby wishes to protect their sanctuary in the Glade of all costs.