Hello Lyn.First thing is......don't panic!! i have both water worms and leeches in both my nature and my fish ponds and they do absolutely no harm at all.Leeches will grasp on to the plant or pot at one end and are generally small,reddish brown and thicker than the worms.They are longer and thinner and do not have the suction powere that the leeches have,in fact they are just like garden worms to look at,only thinner.
The only time you ever need to worry is if you see a similar type of leech actually attached to a fish.The red ones never do.In 14 years of keeping many fish,I have never had any parasite of any description invade my ponds,so they are really quite rare.
