look under springtails on Wikipedia - springtails are collembollans found in leaf litter and other decaying matter on which they feed -
if what you are seeing are springtails they are harmless but... I would change the current substrate and maybe try one mulch at a time to see if the problem alleviates - of course, mulch must be changed frequently anyway -
cypress is usually considered the best choice - I think the topsoil would be especially suspect