More than likely they are oribatid mites. When companies make potting soils they often use forest compost and these little critters live in it. They're not harmful and in fact help break down soil making it easier for plants to absorb nutrients. When I see them in my pots i know that my soil is healthy and thriving. That being said, I grow in containers outdoors so I don't worry about them in my home. If you are growing inside you might want to lay some sticky cards on the soil surface just to keep them in check. Like I said, they're not harmful to you, your plants, or pets.