I do not know what that is, but a drooped ear and sudden fur loss are valid reasons to go to a rabbit savvy vet. Is this rabbit housed with others that may have attacked that portion of the ear?
If it is self induced fur loss, that type of perseveration on an area could definitely be a cause of concern, whether it is a surface issue like a skin concern, or something like an expression of distress
My other thought is something fungal or skin related, but it sounds like this happened overnight? And in my experience fungal infections like ringworm don't pop up in such big patches so suddenly.
Again, regardless of cause, I recommend having your rabbit assessed by a vet as soon as you can. A drooped ear can be a sign of infection or a neuro-related problem.