I had them not long ago. Most of the worms are harmless, although I do think you probably want to get rid of them. Here is a thread on many of the things that can end up in our tank and what is harmful and harmless.
viewtopic.php?f=5&t=25780I had two different types of worms. One I'm sure was planaria and the other I'm not sure. I also had some type of copepods. They were there for well over a month after I discovered them before I could take care of the problem. My hubs was on military duty, and there was no way I could tear my tank down, clear everything and chase around my two daughters.
Basically what I did was empty my tank, throw away the old sand, wash the tank, my filter, filter tubes and everything in it with vinegar water. When trying to find away to get rid of these things without completely tearing down and starting all over I found on a fish website where salt will kill planaria. A guy showed pics of planaria he had gotten out of his tank and then showed pics of the planaria after he poured salt on them. They just shriveled up and died. That being the case, after washing everything with vinegar and water, while things were still wet I covered them with salt. I let it sit for awhile and then rinsed very well. I even poured salt down the tubing for my filter and let it sit. I figured salt would be less harmful to my RES than bleach would be if I didn't get it all rinsed off. My silk plants I washed in salt water and then let them dry out for a month before putting them back in. I have one moss ball, I dipped it in salt water and then plain water before I put it back in. My biomedia I just rinsed well with water.
I had now been well over a month since I did all that, and so far I've seen no creepy crawlers (as I started calling them). I sure hope I never see them again, as it was a witch of a job to get it taken care of.
Good luck.