I also found this white stain, again my husband did not know where it came from~
I took out my trusty bottle of peroxide, I transferred it to a spray bottle for ease of use~
Here it is doing it"s job~
I just allow it to sit on the spot I do not try to clean it off with a cloth as I do not want the stain to get into the fibers of the sofa.
Here is where the stain "WAS"
Like magic it is gone~
I love to use peroxide on stains I use it on all types of fabrics, it is inexpensive & it cleans & lifts stains so easily.
Thank you & Many Blessings~