Molds are a classic thing that grow underground. According to this site
Molds need moisture that can come from water leaks, flooding, high relative humidity, or condensation. And, molds require oxygen, but not light. Without the proper conditions, molds may not grow but become dormant. Then when conditions are again right for growth, they begin to regenerate.
Oozes also don't need light to grow. (they're one of the quintessential D&D monsters). Examples are slimes, gelatinous cubes, and the ooze itself.
So too are fungi (mushrooms). Believe it or not there are vast networks of mushrooms underground that support life above the surface in the jungles. Without this under footing of mushrooms, many plants wouldn't be able to survive!
Moss, Ferns, etc.
According to this site, while not adapting to the dark per se, mosses, ferns and/or liverworts may be in the twilight zone, which are low-light areas where light decreases from the full intensity of the surface. Such organisms are adapted to the very low light regions where the twilight zone meets the darker areas of the cave and like the constantly cool, moist environment provided by the cave entrance.
Roots can grow into the cave. Even though they aren't a separate entity that exists from the plant above the surface, they are still something that grows into caves. Keep in mind that root structures weaken caves, and if there's sufficient weight above the surface, it'll cause it to collapse into the caves below.