heh been so busy I almost missed this one, take a close look and the answer is right in front of you! If they seem to move around in random directions, can flit off fairly quickly when startled., are fairly straight in shape and are less than a millimeter long you've got copepods, some of the tigriopus species will get up to 2000 micron (2 millimeter) but most stay around 700-1200 micron as adults. Very beneficial very easy to keep.
If they're shaped like comma's and look somewhat shrimplike with praying mantis arms their amphipods, these guys get fairly large (I've personally seen up to 4 millimeter) and are also very beneficial!
If they stay on the glass and dont seem to posess much swimming ability, are flat and white and move like worms their acoel flatworms, not as huge of a pest as many make them out to be but not so desireable either. They show up sometimes while cycling tanks and will wane as nutrients balance out. You can siphon them out to help control population as well.
As far as how to keep the critters in good order it's not terribly difficult so long as you have a
refugium and a good rock amount in the display, they'll naturally hide out and breed. Wrasses, many Goby's, pipefish, hermits, etc. will feed on them en masse while most other fish will only occasionally snack on them here and there.