The developer solution is somewhat complex as to the reactions occurring. All commercially available formulas add proprietary goodies to combat hard water. Often, dissolved minerals from the water will precipitate out of solution. All water contains dissolved oxygen (the developer is mainly water). The developerdeveloping agent has an affinity for oxygen and this reaction destroys the developer. The developer is a derivative of benzene which was initially extracted from coal (now likely synthesized). As the developer reacts with oxygen, the developer reverts to coal tar.
All formulations contain a preservative which is sodium sulfite. The preservative reduces aerial oxidation and reacts with oxidized developer to render it non-staining. After a time, the preservative is exhausted and gives up. We replenish developers with a modified formula that rejuvenates. Other solids will precipitate out of solution especially when the developer is near exhaustion.
Most films sport an anti-halation coat made of dye. Some motion picture films are back coated with a Rem-Jet (removable jet black) make of lamp black. The Rem-Jet is held in an acid plastic that releases in an alkaline solution. It is not likely that the sheet film you are developing has a Rem-Jet.