Mites are often a common problem with fruit fly and springtail cultures. They probably got inside the viv. when feeding either one of them. I would think they were breeding in the cultures not the viv. They will provide the frogs with a "Little" snack (lol could resist.) If they are in the cultures they can cause a lot of problems for the culture and eventually end in a culture crash if not kept in check. If they are in your fruit fly cultures the best way to keep them in check is to never keep cultures older than 4 weeks. When making new cultures always start with a new hatch and avoid dumping the flies inside instead gently tilt the container and tap them in avoiding the debris from the bottom of the culture. Not sure about springtail cultures i have never had a problem.