To cause you to look into his Sharingan eye.
u r right i think so the bee hive like shape confuse u when u see and that confusion make u see his sharingan and then he can easily use his genjutsu
i got theroy of madara wearing mask i told it before
the madara told that the uchiha clan and senju clan fought and uchiha wins madara was leader of uchiha clan and he took his bros eye
and that war was held about 100 years ago,then madara was young (young may be 20-25 years old)then madara is 125 years old(may be),so maybe he is hiding his wrinkles on face
or maybe he got a swear cut on his face in that war maybe hiding that cut