Yes, IMO, pipe cleaners CAN damage the pipe, I've seen it. You have to work on it though. If you push the cleaner in till it hits the far side of the chamber and twist them, it will act like a drill bit and drill into the briar. You'll drill right through the chamber if it is kept up. Remember, most times you use a cleaner the bottom of the chamber is damp, and soft! I always spin my pipe cleaners to get the best cleaning but when it hits the far side I back it out about 1/8 inch then twist the cleaner.
As to the OP, looks just like cake to me, nothing to worry about.
Thanks for the info. Yeah, I don't twist or go rough on it, so I'm thinking this was already there as I've cleaned every pipe the same way and never noticed it. I'm thinking it's either cake or was already there. Maybe I'll sand it some and see if it's cake