Uhm.
The reason it is there is because of the floors "reflection".
The floor is half transperant so that you get the illusion of reflection when you look at the floor.
This is why everything except for Link is reflected, and that is because the mirrrored one below isnt complete, it is just parts of the temple.
I am 100% sure there isnt any collision for it.
That would just be dumb and isnt required whatsoever.
We shouldnt be able to get there either, it is not like the gravity turns around when we are below
You can get a clear view of it by walking into a 90degree corner that points towards you, and then use the camera view before it pulls you out.
Links head will be inside the wall, and you can see through it so that about half the screen can see below.
This trick is also used to hookshot through some walls, you just stand inside of a corner.
I thought it was quite obvious