Do you have the Bridge you can go to? If yes, choose find another way in on the Bridge screen to visit the cave, the 5 Glowshroom should be auto picked up after arriving outside of it, if not, just choose to search/look around to find them, then you ask the white Lizardess in the Inner Temple inside the Ruins or the Witch in her hut about the cave and pick yes to have her create the item needed for that, after doing so, equip it in your equipment screen (Click/Tap the Wooden Sword icon) and equip the Amulet you got from either of them before going back there and you should have the option to enter it, it need to stay equiped to explore the underground each time you go in there.