Black Scum

[Charlie B]

I have just started to use a thermobox for a fermenter with a stainless coil cooler fed from a chiller. I have done 2 brews in it so far and they have both had some black scum around the leftover yeast ring. I have used brass fittings on the lid to secure the coil but it shouldn't come into contact with anything unless the yeast head rises that high. What could this be? I will try and get some pictures of the stains that are left on the stainless after cleaning but I don't have a picture of the scum until I next brew.