Um I maybe tripping… but u guys have possums under a millimeter? Thats weird lol. I believe u mean some kind of pests? Anything small NOTICEABLY moving in my soil gets a Food Grade DE treatment and possibly a peroxide spray (depending on the infestation level).
From the pic, looks to be root aphids or some critter of the sort. Definitely bad guys! If u dont want to resort to chemical warfare, predator mites/lady bugs/some spiders/ and possibly mantis are ur friend.
But food grade DE would cut the lil buggers exoshells and make reproduction impossible.