Honestly, I keep reading Madara has got a "headband", a "forehead protector" or things like that.
It is a HORN. Seriously, look closely. It emerges from around his left ear, crosses his forehead and rises up on his right side. The other smaller horn is growing on top of it.
It is similar to Obito, who also had one smaller and one larger horn, only that Madara's is much bigger (since he got more powerful).