It accumulates over time on the glass, and sometimes over plants and rocks that are exposed to high power lights, or are placed too close to a light source. It appears as a thin, green film that spreads and spans across objects much like a spider web. It appears and feels like a thin, solid veil, but as can be seen here at 400x magnification, it is much like an algae - probably the reason for its common name of Blue/Green Algae. Its cause, too, is much the same as that which causes algae - too much light and too many nutrients. In an aquarium, frequent water changes will help eliminate it.