What is that dark black green sticky residue on my heating plate after descaling?

Don't panic, there is nothing wrong with the unit.  This is simply telling you that you have operated either the Auto or sterilizing mode when your true intent was to descale the unit.

The dark green sticky residue is just a reaction after the citric acid being overheated and burnt. To clean this residue, all you have to do is just add another 100ml of distilled water to the heating plate, make sure you operate the descaling mode again, the residue should come off completely.

Prevention: Make sure you press the descaling button.

TIP: also make sure you do multiple fresh water rinse after descaling, though citric acid and vinegar are great non-toxic cleaning agents, it tends to cause pitting to all stainless steel surface over long period of exposure or not rinsed thoroughly.