The red one is the idler, it receives no power and just turns in response to the green one. If it is not turning, there are a couple possible solutions:
1. Your teeth on the gear (green circle) are clogged with filament or damaged and are not gripping the filament. Based on debris shown, that appears likely.
2. Your spring tension is not tight enough. Try to tighten it.
3. Your hot end is clogged, not letting any filament thru even if everything above it is right.
4. I have had cases before where there was nothing wrong with the above and I still couldn't print with the stock gear because even on a newer printer, it is just a piece of junk with very shallow and dull teeth. Consider upgrading to an E3D hobb goblin and you will also get better resolution. There is a thread about this.
Try fixing 1 and 2 first. Also, it is recommended to print the lawsy MK5 extruder while you still can before the acrylic inevitably breaks, leaving you printerless and without support from solidoodle.
SD4 w/ RUMBA, E3D Volcano, all bearings, glass bed
