If they were root aphids, you most likely wouldn't see them until it's too late to save the plant. Be glad they're not root aphids!
They look like Hypoaspis mites to me as well. As said previously, Hypoaspis mites are included in FFOF as a means to control the young larvae of soil-born insects, such as thrips, springtails, fungus gnats and root mealybugs. When there are no insects to consume, the mites will feed on plant debris and algae, causing NO harm to the host plant. Hypoaspis mites only live in the top 1/2 inch or so of the medium. Any medium treated with lime or pesticides should NOT be used if you plan on innoculating the medium with these predatory mites... they will die.
There is one thing about the introduction of Hypoaspis mites in professional made mediums (such as FFOF) that most people don't realize. Another species of mites may be added as a food source for the predatory Hypoaspis mite to feed on. To check for either of these mites a 10-15X magnification will be needed to see them well enough to determine what they are. Predator mites are tan and move quickly, compared to the food source mites, which are white or translucent and move slowly.