I know that these are not spider mites but I had a problem with spider mites during my last grow which was indoors. I sorted it out by mixing few drops of neem oil and dish soap in water and giving my girls a good shower just before lights out, because the oil will magnify the light and your girls will burn. I did this every other day for a week, then I put a few drops of the neem oil into the feed every week while in flower and problem sorted.