The word mites is initially scary for gardeners who think spider mites! There are, I've learned, some beneficial mites that help to decompose organic matter. These mites are only a problem when there get to be so many that they interfere with the worms. The fix according to one SITE is to buy yourself a watermelon or cantelope. Eat the good part, and use the rind as a lure for the mites. Any member of the squash family are good lures for mites.

Lay the rind pieces on top of the bin overnight. In the morning gather up the rind pieces (which should have mites all over them, and rinse them off with warm water. You can then place the rinds back on the worm bin again overnight to remove more mites. I would treat both bins that show mites.

Since you mentioned the middle tray is finished, you might want to remove it from the stack to do the water melon treatment. Easier to do rather than stacking and un-stacking. I assume the bottom tray is finished too except that it still has the worms and cocoons.

Maybe you should place the tray that is now on top but has all the scraps and bedding right on top of the one with lots of worms. It will be easier for the worms from the bottom tray to reach the food. They were supposed to migrate to the bin right above them anyways and we lure them with food.

Coconut coir is not absolutely necessary. You can use shredded newspaper instead and torn cardboard. If you have a shredder that can manage cardboard, that's even better. Coir can help but it's not free. Instead of coir, I sometimes use saw dust pellets that PetSmart let me have. This is the stuff/bedding that they replace from their small animals cages once a week. Ask them when they do this routine and I picked up around the time they're done with their chores. Use just a few because the liquid/moisture from the worm bin will be absorbed. I use it mostly when my bin is too wet.. The presence of lots of mites is a possible indicator of too much food and/or acidic condition.

I have the worm factory 360 too, had it for a couple of years. Not sure what your schedule is but I have had good luck with using only 3 trays at a time and rotating them every 30-45 days, only adding food to the top tray every 2-3 days, only accessing the bottom 2 trays at most every 2 weeks just to stir the casting/ compost. I have done this and the mites aren’t as bad. I have had my issues with mites and stressed about them when I started. They are part of the process. Yes the melon rind works but they come back. Once you build up 3 bins and don’t touch the lower 2 the moisture kind of takes car of its self. Just add food to the top one and you should only have clean the collection tray when you rotate as long as your not always adding super wet food. If you are add saw dust to their diet ( frees from Home Depot if you ask at the cutting station (might get composite dust in there)).

I would say you should keep your bins as is. if you have core great if not shredded news paper is fine. place food like it says. Under the shredded paper or core and rotate each corner each feeding. The second bin will correct its self. I’ve found less is more with the 360. The less I bug them the more they do. I know when I first started I checked that thing 2x a day. Now I check it 3x a week and they make more castings. And I have less mites.

Good of luck and just FYI I‘ve Had a bad bin before too. Just didn’t produce much, but since then I’ve backed off and done less is more with better results.