Don't aggressively sand the door. This will ruin the fiberglass surface. It is odd that bleach won't kill the mold.
What you can do is remove the door and set on some padded saw horses. Now use a garden hose and a stiff nylon scrub brush to remove the larger pieces of mold, if there is any. Flood the surface with fresh straight bleach and continue scrubbing. Allow the bleach to sit on the surface for several minutes and reapply if it tries to dry out. Rinse off with the garden hose. You can also try commercially available mold cleaners or TSP to remove the mold then use bleach to kill anything still on the surface.
After the mold is finally removed use a fiberglass safe paint stripper to remove the stain.