Yea, thats rust 100% no doubt about it. I actually didnt realize Stainless could rust until I went elk hunting in some really wet damp weather (imagine that) and when I got home noticed the same exact thing as you picture.
I removed it as quickly as possible, and treated the bbl with some rust prevention spray. Hopefully your bbl isnt damage yet, It doesnt look like its deep or bad enough that it cant be rubbed off without leaving signs it was there.
Best of luck