*long suffering sigh* I wish people would relax about the worms. Fingers are tired today, I'll post the short version.
99 times out of 100, the worm you see is not just harmless, it's beneficial. Unless you have a) a bobbit worm b) a carribean bearded fireworm AND gorgonians or c) a worm over a foot long... it's fine.
As for a and b, wikipedia has good pictures. Your worm just happens to be your tank's best friend.