人間とは月のような存在だ。他人には満月を見せていて、人という光にあたって誰もが輝いて見えているが、その裏側は黒く、暗いものだ
is 人という光 a contract form of "人というのは、光" or it means "light called humman"?
This 人という光 means "light called human" or "light that is human". The sentence says that every human has an external-facing side that looks bright like a full moon because it's "illuminated by (the light called) other people".
他人には満月を見せていて、人という光にあたって誰もが輝いて見えている
They show the "full moon" (side of themselves) to others; everyone looks bright, illuminated by the light called people.