No one has actually seen it, but a fairy interpreter asked a fairy and they described it as just a cloud of smoke and once in a while you trip over something in the floor.  The fairies purposely leave their trash in the floor because they think it is funny when people trip over them.
Inside the fairies church, Inside Church
bee
fairy
I'm a fairy that lives in the forest. I like to fly around with my siblings and play games. I try to hide from humans. If they find me, I'll grant their wishes.
Waaah! A bee! Wh-what are you doing in our church? I though I smelt pollen! Why is it so smokey in here? I cant see a thing Oh! You poor thing! This is the Fairy Church! It is filled with the scents of our forest home! My but you are far from your native hive, are you not? I am very far. You forest home smells so lovely. Are there flowers of the sort around? Oh... flowers? Giggle! Indeed! Do you not see them all over the floor? No, I can not see a think through all the smoke, though I do hear a cricket chirping in the distance. Oh, Bimmy? He's my good friend! Hold on, dear bee! Let me help you! You are much kinder than I would have taken a fairy for! I have heard some not so nice things about your kind. But here you are, helping out little ole me! BUZZZZ Of course! Why not? Now here! Take this... enchanted feather! Wave it around real fast and the smoke will leave your sight! *waves feather around* Oh my! This place is a mess! Why is there trash all over the florr? Oh! Well, its easier to keep the fog around than to keep the place clean! You speak the truth!