Jobo Residue: What is it and how to get rid of it?

So I have recently switched to Jobo from stainless steel in an effort to maintain consistency, as well as for occasional color processing. I am mostly using black and white chemicals though -- Xtol mostly. Unfortunately my tanks are now showing a bit of brownish/grayish residue on the interior walls, as well as the bottoms of the spindles. What is this? It seems to wipe off with a tissue or gloved finger but does not seem water soluble. The tanks are new, so I know it is not some odd leftover from a previous owner...

I wash the tanks only in warm water and do not use photo flo in the tanks or reels. So, is it time to break out the soap and water, or better yet, give them a run in the dishwasher? Anyone know what this stuff likely is? My guess would be some leftover silver salt, but I really have no idea.

I always thought it was elementary silver. On my Jobos, I can wipe it away from smooth surfaces like the inside of the drums, not from the spirals. The latter got a bit cleaner when I put them through the dishwasher at low (50° C) temperature.