What is this stuff?

[brewit2it]

Hello everyone,

Hopefully someone has seen something like this before and can help me out.
What is this black stuff forming on the inside of my still head?

I've done some reading and I know that sulfur compounds(copper sulphate) will form, but this just seems excessive. It even seems to be flaking off and coming out in the distillate. These pics are from after I cleaned the pot still head with a phosphoric acid solution and did a stripping run of an all grain wash. Since my all grain washes have been coming over dirty(tiny brown and black flecks) lately I've been running them pretty slowly. I can wipe most of it off with just a paper towel, but need to scrub to really get it clean.

Thanks.