I've seen that
I have a bunch of new airline carpet..enough to carpet a B777 for real. It does the same thing when it gets wet. I use it for hangar floor, garage floor,ect, and when it gets water spilled or dripped on it, white rings/powder will form. I pressure wash it, and it looks like soap foam big time. I assumed it was a fire blocking substance, and oddly, in 42 years of working airliners and seeing "in service" carpet get cleaned with Stanly Steemer type equipment I never noticed it.
8A7 / Advance NC