Eww. Does it have a flat head or does it just look that way?
It sort of looks like these worms I had one time that were living in the gravel and they would stick out and wave around creepily. Someone told me they were tubifex worms, but they didn't have flat heads.
I'm pretty sure it's a tubifex worm, then. I don't think you should be worried because they are harmless. Personally they totally creep me out. So much so that I carried my entire 2 gallon tank outside (I was only using it for a plant tank while my 55 gallon got set up and there was only one plant left) and it's been out there ever since (3 months). haha. The worms seem to be dead, though.