It's a good thing. When I started my first 55 gal
aquarium, I had a bunch of live rock and very few critters. I had coepods, amphipods etc.. crawling arround like crazy. I thought my tank was infested with some sort of parasite.
Sometimes they get cought in your skimmer and in your filters. No big deal. My wife like to save their little lives and put them back in the tank. They're cool. They live in the nooks and crannies and help clean up a bit.