In some areas of the Underdark, phosphorescent lichens provide a natural light in the darkness. They grow in pockets in every layer of the Underdark.
Unlike phosphorescent mushrooms, where large clusters can provide enough of a glow to brightly illuminate a cavern, the light from phosphorescent lichens is fairly minimal. A hominid without other senses might have enough light to read by if they press up against the lichen, but otherwise the lichen serve little more than to show where the wall is, or show a pathway down a tunnel.* Characters without other senses to help them would get minimal help from the lichen: other characters are seen mostly as shadows, and opponents in combat are treated as if they have half-cover.
Phosphorescent lichen do not have the sunlight hypersensitivity that phosphorescent mushrooms do. They will lose their illumination in a few days of regular exposure to sunlight, but will regain it after a few days of darkness.
* DM’s Note
Players can think of the lichen like the glowing exit sign in an otherwise dark theater: enough illumination to see where the door is, but you wouldn’t want to rely on it if you were in a fight with the ticket-taker. Especially if the ticket-taker had night-vision goggles.