Looks like sooty condensate to me. I used to see that all the time when I was in the business of motorcycle repair; even brand new ones did it.
When the engine runs right, the exhaust is full of water-vapor, as a byproduct of the energy conversion process. You can see it every morning in cool weather when you warm up your car.
Your exhaust system is running too cool for too long. Suzuki solved that with dual wall pipes, and special clamps with impregnated wiremesh inserts, and underslung mufflers.
For you, I recommend ;
burning out the exhaust baffle
a headerwrap on the frontmost vertical section,
RTV sealer on the joints
and set the idle-speed higher.
The idea here is to keep the exhaust hot enough so the water vapor exits the system, instead of condensing and running back into the low parts. After the muffler fills up with water, it has no place to go but downhill, unless
you blast it out by reving it up, or
drill an exit hole, braze in a spigot, and rout the near boiling water away with a silicon hose.
