This looks like old Four-lined Plant Bug feeding damage, and the insects are now finished with their life cycle for the year so will not be coming back for another round of feeding. No intervention is needed because the damage they do does not seriously impact long-term plant health. If too unsightly, you can pluck off any individual leaves with heavy spotting, but no other action is needed (no insecticide, and no more fertilizer than you may already be using if the plant is growing in a pot). Mint is such an aggressive grower that it should soon cover-up the old damage with new growth.

Miri