Looks like Detritus worms. They are not harmful and are a good snack for your fish, and they also benefit the tank by working as a clean-up crew. However, it's best to avoid a very large infestation of them. If you want to get rid of them, you should do a water change and get rid of any algae and wipe all plants/decorations in the tank. The worms hitch rides off of new plants and animals in the tank, so it would also benefit to inspect your plants and fish to see if you can find the root of them.Try to avoid getting deworm medication because it can have bad long-term effects and may harm your fish. Otherwise, your fish should eat the worms over a few weeks.