[Pumilo]

Probably springtails. They start up for apparently no reason sometimes. Probably come in on a plant or the dirt stuck in the roots. When they come out, grab a straw and gently blow on them. If they are springtails, they will jump. Some species jump several inches. My giant blacks can jump 5 or 6"! Another clue is that springtails are usually much faster and more "chaotic". If they were isopods in any sort of density, you should see ones big enough to be identified as a "Rolly Polly".
Both of the bugs I've mentioned are good to have.

 

[Pumilo]

In cases like this the mites should be identified as I've had cases like this when I've found a freshly dead frog covered in mites only to have those mites turn out to be free living detrivore mites that are commonly found in the enclosures. In those cases, the mites don't kill the frog, they are simply the first decomposers on the scene. If there were/are blood sucking mites in the enclosure then you should find them attached to the frog(s) while the frogs are still up and active or lesions showing where they are embedded in the skin of the frogs.

See this abstract for an example of the issue JSTOR: An Error Occurred Setting Your User Cookie
Pathology of cutaneous trombidiosis caused by larval trombiculid mites in a wild Lesueur's tree frog (Litoria wilcoxii) - [email protected]

Almost every viv, maybe every viv, is going to have mites sooner or later, probably sooner. The vast majority are going to be harmless to your frogs. Scavengers and decomposers can easily get a bad rap simply by doing what they do best. As an example, to this day, many reefkeepers still mistakenly believe that bristle worms are predators. In reality, out of the hundreds or thousands of types of bristle worms, only 2 have ever been proven to attack live flesh. There feeding response is triggered by dead, decomposing, flesh. So a reef keeper fails to meet the requirements of his prized Maxima clam. Clam looks fine one day, but is dead and covered with bristle worms the next. Reefkeeper believes that the worms killed his clam and goes on a "worm rampage", trying to capture and kill every worm in the tank. Little does he know that the worms actually saved his tank from a major crash. A huge chunk of meat (the clam) rotting in the tank, would have caused elevated nitrite and ammonia levels, causing more animals to die and levels to rise more, eventually crashing the tank. His thank you to the worms...death!
Maybe a sidetrack, sorry. I just thought it would be some more info backing Ed in that most mites are really harmless to your frogs. I use contaminated cultures to feed my pumilio babies. Mites are like candy to them!

 

[Pumilo]

Mites do not jump. Springtails do. Springtails are a valuable and natural part of our vivs that should be left alone or encouraged. They work as janitors, cleaning up your frogs poop along with molding and decomposing leaves etc. They are a natural source of food that even most larger frogs enjoy hunting.