the mature adults are harmless to your plant. It is the larva that are detrimental. They hatch from eggs laid in the medium and feed on the root system untill the emerge as adulrs.
Dry that medium out. It's too wet, you're keeping it too moist. Water it thouroughly when you water, then leave them alone untill you notice drooping.
A layer of playground sand an inch deep, on top of your medium will help a lot.
Watering from the bottom, rather than on top of the medium will help too.
If you can find a product called "diatomaceous earth", it is fantastic for ridding your grow of any soil/medium dwelling pests. It isn't a poison nor toxic, but should be used with some care.