Foiled pack of apple chips
Master built xl at 215
Salmon wet brined and pellicle formed.
Vent fully open
This is usually caused by soot falling from the top of the smoker as you knock/vibrate it as you open and close the door. There are a few ways that I avoid this when I cold smoke.
First you should give the inside of your smoker a good brush out with a stiff brush and a damp cloth before starting the smoke. This will minimise any loose soot on the top of the smoker becoming loose and dropping onto the fish.
You can smoke the fish skin side up. That way any soot that does fall will only mark the skin side. This may not be an option though if you are hot smoking
The third option is to place a piece of foil above the fish - maybe resting on the shelf above or on a rack. This will catch any of the falling soot before it can fall onto the fish itself.
I'm not sure what you have there, but I have had what Wade is talking about (one time).
However mine was black chips that looked like black paint chips.
Ever since that time, I brush any loose peel-looking black stuff off the ceiling & sides & back before preheating my MES 40.