Of note - the fly I am referring to have an all-black body.

The worms do need help from mites to beak down food for them but not in the amount as on your pic. I'm guessing that your bin is too wet and you have too much food in there, that often leads to too acidic condition. That is also another reason for having fungus gnats infestation. I'm pretty sure your small flies are fungus gnats. If the mites get too many for the worms to bear, the worms might try to leave.

Try to remove any extra food. Sprinkle crushed/powdered eggshell through out. Add LOTS of shredded newspaper on top, NOT mixed. By piling shredded newspaper on top, that should keep the gnats inside the bin if you keep the lid on. Fungus gnats usually go for glass windows and doors and that's the time to get your vacuum cleaner out and suck them in. Some people use that bright yellow sticky paper you buy at the nursery, Place some in and around the bin.

If you have extra room in your bin, you can do the following to help along extract moisture from the bin contents. Crumble single sheets of newspaper and stuff them along the wall of the bin. I use the 2 ply corrugated cardboard but with a fungus gnat problem, I wouldn't use corr. CB because fungus gnats breed between the CB layers. Good luck with your battles.