Yep, almost all the non-tubeless conversions of mine and even a few Tubeless Ready tires have done this. Some stop after a while, some do it the entire time of running the tire. None have ever "melted" my sidewalls.
thanks for the input, in years of tubeless honestly never seen it before. the tyre is degrading, as above black "greasy film" residue. i thought it would be a case of throwing it away, but i'll keep it on there.