I have never seen that before and I work in a tire shop. I have worked on many vw's and Audis and yours is the first I have seen that in. Although you said you used fix-a-flat spray in the tire and sometimes it does that. When it separates from the aerosol it creates a bond with the wheel and forms that brownish coating.
P.s. For anyone who uses it, fix-a-flat and any spray that says it'll stop a tire from leaking is wrong. It doesn't work, it actually causes more internal damage to the tire which will cause failure of the tire faster. So if you use the spray, you should replace the tire for your own safety.
DANG....thanks for the info... guess maybe it was the slime i put in it... yea i didnt know either so i finally went to get the tire "fixed" and they said here its illegal because its flamable etc. so i had to swap the tire anyway...
buuut i went from hankooks v4 to v12 evo...muuuuch better