That's a symbiote which shrouds him and it consists of 100% pure black metal. It's clear because the human devices and our eye is unable to register the colour which exists in a separate dimension, parasitic in nature. We are its host.
By the way, the guy behind 3 should know that he has an small, but extremely loyal and dedicated cult worshipping him in northeastern Brazil for years now. Can't tell how much he owns it to this picture rather than his musical merits, though.
That picture has changed my world from the moment I saw it. Fuck church burning, grave desecration, murder, corpse painting: all that is utterly trivial and meaningless now. Thank you, 3, for opening my eyes to the blasphemous and occult meaning of putting a misanthropic and bleak plastic bag in my head!