Sounds like mites. Occasionally you'll get substrate that has them or they find their way into the enclosure from somewhere else. Even from feeders like crickets carrying them in unnoticed.

You'll have to remove everything and clean it. Throw out the soil and replace it with new. Bath the frog and rinse it off to make sure none of the mites are hiding on it. Check any furniture as well. Even the water dish.

Should be fine. They were most likely harmless soil mites.