definitely not mice, i've raised enough in my day to know what those droppings look like... those shitty things look almost like worm castings.. have no idea what they are though, check the area at night with a blacklight, maybe you'll catch the perpetrator... the little white things look like larvae - fly or gnat larvae..

the poo is good for the plants, so as long as whatever is making it is benign, you're good!