Hi Jane, and welcome. You have an infestation of bugs on your mint. Try spraying the whole plant with a solution of soapy water, about 3ml. to a liter of water in a spray bottle. Be sure to spray under the leaves and the stems and soil under the plant as well. Leave it on for a little while, then rinse the whole plant off with water.
Your mint will probably be happier growing outside through the summer months, just be sure to give it lots of water. Keep it in a pot or it will tend to spread and take over your whole garden. The natural predators will take care of the bug problems for you out there.