I had the same "Oh my god! What the hell are those????" moment when i first saw copepods, and then later planaria in my tank.
Funnily enough, just about every single question one would need to ask has probably already been argued over and answered in several different ways if you look around this site, yet, very few (apparently) use the search function (very top menu bar about the middle FYI) , Or have a look at the sister blog
Having said that, how boring would it be if no one asked any questions?
I found a slightly increased cleaning regime reduced the harmless to axies copepods and planaria, and the planaria were totally eradicated later by an aptly named product - "no planaria" (which i only used because i wanted cherry shrimp i my tank and plarnaria kill shrimp)
Since then i have put white cloud minnows and guppies in the tank and i think they patrol around and keep those sort of bugs managed too.
Bottom line, they are harmless but a good cleaning regime will keep most bugs and tankmates down to a reasonable level. I think those sort of things (copepods, snails, planaria, algae) are only detrimental to your tank if they get out of hand and overrun it, otherwise it is a sign of a healthy ecosystem
Bren