You probably have some species of tortoise mite(Are they red or tanish)???????? If so then you have nothing to worry about, as they dont harm your roots or the plant in any way, they merely feed on decaying Organic matter, and are quite beneficial in a soil based TLO grow, though in coco not so sure.....If not that then they are probably Hypoaspis mites, which are not bad either as they are predatory and feed on fly larvae and other small invertebrates, helping to maintain balance in the soil ecosystem........ Now if your populations of mites gets too big,(which sounds like your case for sure) one of the best ways to control there populations big time is this......... Take some watermelon rind or any kind of melon rind and lay them on the top of the soil, flesh side down, and in your catch pans as well. Let the rind be for 24 hours. When 24 hours passes, get a bucket and place all the rinds in the bucket. At this point they will be absolutely covered in mites. All you do is take the bucket to the sink and rinse the mites off the rind, and then repeat until very few mites are left on the rinds after 24 hours. Another thing you can do(Which decimates the mites, and is also very effective against springtails, and symphilids.) is steep some organic tobacco in water for a couple hours, like a cigarettes worth per gallon of water. Take the gallon of nicotine water, and mix it with 1.5 Fl oz of Organicide(fish oil, sesame oil, and lecithin) and apply it as a soil drench. 30 minutes before you apply the drench, you should lightly water your plants to moisten the medium. Good luck brother!!!!