Those are probably phorid flies. Chuck the moss. Let everything dry out for a few days and then replace it. They are harmless, but will always come back as long as they have heat, humidity and food.
Two other possibilities are fungal gnats, which are tiny flying bugs half-way between skeeters and flys; or springtails or snow fleas, which are tiny fast-moving greyish bugs.
None are harmful, but can be troublesome to get rid of. I found that using a 'bioactive substrate' (it is discussed in other threads here) with rolly-polly bugs (also called isopods, wood lice, etc.) helps since they prey on the eggs and young bugs.
We went from lots of both to almost none of either after about 2 months of the bioactive substrate and about a half-dozen isopods getting settled in.