What the hell is wrong with this bend pipe ?

Bike - Triumph Speed 400 Does anyone know why this keeps happening? I’m honestly fed up because this is the 4th or 5th time it’s shown up on the bend pipe. The first time I noticed it, I scrubbed it off using shampoo water and a Scotch-Brite pad, and it worked—everything was clean. But after some time, it came back again. When I first saw this, it was during the rainy season, so I assumed it was caused by water droplets. But now, there’s been no rain in my area. I cleaned it thoroughly last month, and it’s been dry since then, yet it’s back again. What should I do now? Any suggestions or Jugaads. Please let me know!