With power comes visibility.
Filters do not make a laser more visible.
For 5mw lasers, you need fog, a smoke filled room, a blizzard at night, or run the vacuum cleaner backwards and fill the air with dust.
Otherwise, you need laser power or a fog machine.
A 5mw green can be seen indoors in subdued lighting. (Assuming you don't run a HEPA furnace filter)