It should just be spray painted.

The last time I bought bananas in an underground passage, I found that there were many small white spots on the bananas and it was difficult to remove them. I thought they were drugged. The seller couldn't explain what was going on, but later he remembered that the tunnel walls had just been spray-painted and the bananas were stuck on them.

I guess it is the same for you, there is spray paint on the side.