Looks like soot from exaust. Particles travel through the wiremold from the RE where the harness passes into the engine bay.
Is it more prevelant where the harness trunk-line branches out? We had soot. Leaky buses.
I'd keep the insulation. Maybe add more foam, if you're able. Closed cell will encapsulate what's there. If it were mold (unlikely), just being present, while it's being pulverized, seems less healthy than the alternative, imho.
"A man is worked upon by what he works on. He may carve out his circumstances, but his circumstances will carve him out as well."