Urban Rooftop Green Spaces

Recently relocated to Seattle in an apartment overlooking Benaroya Hall, and my view of their mossy(?) roof in comparison to other “cleared” rooftops in the area has me curious if theirs is intentional rather than accidental? Any locals want to help me out?

Loving the thought of a springtime bloom in my future, but imagine complications like weight, drainage, etc. might be why the vegetation is limited to the moss/grass?