Howdy,
I think Peanut_Power is on the right track, suspecting nematodes (roundworms). Watch these things closely for a few min and see if they move. If they do, they're most likely planaria (flatworms). What fish do you have in your tank, and what filtration? Excess planaria are mostly caused by underpowered filtration and/or too long water change intervals. They cause no harm but are not pretty, either. Gouramis and bettas love them. They can be battled chemically, but I strongly advise against that. Frequent water changes and better biological filtration will take care of them
HarleyK