The little guys are red planaria. They really aren't that harmful except that they reproduce quickly. In some cases, they reproduce to such large numbers that, while scurrying around on your corals, they actually block out light to the corals. That's how they become bad for our tanks. Amazingly, a lot, if not most tanks, have them in small numbers. I've read where low flow can cause an outbreak and something as simple as increasing flow can take care of the problem. Some natural predators of these are 6 Lined Wrasse, 4 Lined Wrasse and possibly Dragon Fish.
They can be treated with Flatworm Exit or Prazi Pro, but a word of caution...when they die, they become toxic. If you have a large population of them, the amount of toxins they put off when they die can wipe out your corals. However, if you have a small population, you're okay to treat them. Let me rephrase that...with a large population, you can still treat your tank, but do you have to siphon up all the dead bodies and do a major water change. See below for techniques to siphon them up
Before treating, you should attempt to get as many out manually as you can. A couple of ways to do this is..
#1. using a small air hose with rigid tube on end, siphon as many as you can find out...pain in the butt!!!
#2. Using a filter sock on the end of a larger siphon hose, or on the drain hose from tank to sump (if you have one) syphon out water through filter sock. Filter sock will catch them and critterLESS water can be put back in tank.
Do some google searches and research here on RF for Red Planaria. You'll find lots of good information. Here's a couple of links to get you started.
http://www.melevsreef.com/flatworms.html
http://www.wetwebmedia.com/flatwormfaqs.htm
