Developer dregs
I find a sludge at the bottom of both my Kodak D-23 and my modified Ansco 17M (modified with 2x the sodium metaborate and 2/3 of the hydroquinone). After I replenish, using the overflow method, I cap the bottle, and rotate it several times, to mix the replenisher in. Both developers become a veritable "snowstorm" of the black precipitate, but it settles out and I have never had a problem with it getting on film when I develop, partially because I am careful when I decant the developer into a beaker, before pouring it into my developing tank.
You can try filtering the developer; I do that, using the same filters I use to make my coffee. I'm guessing that the sludge might be some kind of sulfite, or it could be as a result of the rather hard water I use to mix my chemistry. I don't know for sure; I'm not a chemist.