My Jans Best Mulberry Plant

has like a Confection Sugar coating around some of the Stems, I wrote Jan about it and she never seen that happen to her in Florida, I am in zone 9a, this is the second summer that it has showed up. I spray it down good with Nimm Oil, let it soak a while then pressure it off using Nimm Oil Mixture... I would just like to know if its a bad problem or not or just what is causing it. ((Photo Included here)