Have you ever seen a person or creature out of the corner of your eye beside the road or in the forest? Then to turn in that direction and only see a piece of wood or rock. What I learned years ago is you just saw an elemental.
They are the spirits of the natural world who can be good or evil. Generally in my interaction with them they are more benign. They didn’t cloak themselves fast enough and I caught them for a fraction of a moment. Periodically, I see a face in wood like this live cedar today. Another form is in driftwood or dead branches. Captured in time this is a glimpse into their world.
Perhaps my interaction has been good since I give them respect as guardians of the forest or water. There are other stories of them not being so nice.