G'day Carnethy,
It's just dirt around the seal between the sliding part of the stabiliser and the tailcone of the aircraft.
There is a fairly large flat plate which moves with the horizontal stabiliser and covers the opening in the tailcone where the mechanism for moving the tailplane is located. You will also notice a large flat area on the actual tailcone itself.
Have a look at this pic and I think you'll see it more clearly. I just noticed that the aeroplane is taking off with the APU running, the intake on the upper RHS of the tailcone is open.
Photos: Boeing 767-338/ER Aircraft Pictures | Airliners.net
Regards,
BH.