Wizard is right I think, that looks like a Sciarid fly to me. They hang about laying eggs in the soil and their larvae eats things in the soil and will munch through roots when the population gets larger and food becomes sparse. You have several options.
1) pyrethrins, you can safely spray around the tops of the pots. It doesn't hang around for long so requires a few applications but will eventually kill em off. Spray at three days intervals for a week or so until they gone.
2) bacillus thuringiensis, a soil born bacteria that crystallises in the larvae stomach. Works fast but it's expensive.
3) nematodes/predatory mites, will seek and munch but takes about 7 days to kick in. Good for larger grow rooms or outdoor grows.
4) gnatix/soil dressings, these you add some to the top of your pots and it makes the pots undesirable. Some products like diahydra and gnatix claim to activity control because the larvae can't traverse the media.
That's a brief overview of some things to consider. There are other methods but these are your most common. I just cleaned up a pretty bad infestation with tanlin. The active ingredient is option 2 and it worked fast. Good luck buddy

Those who can make you believe absurdities can make you commit atrocities.