Planarians. Not really a huge problem, as alot of people end up with them. They often feed on dead plant matter, left-over fish food, or fish waste. Some Fish will eat them, but I've never known it to be harmful.
Vac the tank, at least, to get rid of a lot of the critters and their food, and 50% water change tends to help. If you want them gone RIGHT NOW then yes, tear down the tank. If you're okay with them getting naturally cycled out, just keep feedings to minimum and Vac the tank, and change water 50% every day.