Mossy Glen Cave is a small bear den accessible via Dayspring Canyon. When you reach the point along the path where you can first see Fort Dawnguard, head uphill on a path to the north to find the entrance to this cave.
Upon entering the cave, you will find yourself in an open area containing a skeever. On the second ledge along the wall to your left, there are two unlocked chests with minor loot. Further into the cave, there are four leveled bears in the northernmost room. After defeating the bears, check the west side of the room for another unlocked chest containing minor loot. To the east side of the cave, a narrow tunnel leads to a separate exit out to Dayspring Canyon on the other side of the fort. Bleeding crown and white cap mushrooms can be found throughout the cave.