Depending on the age of the
engine I would not worry too much as it is likely combustion by product but may indicate a bit of
oil getting into the combustion process (valve stems, piston rings)
The fact it is showing up around injectors indicate the copper washers may be deforming or the injectors need torqueing.
Another alarming place this residue could be accumulating is in the
exhaust elbow/muffler. Id take a look at them if it were me.