Planaria!
Guys, you have it all wrong! These are Planaria, a flatworm that usually come about in tanks where there is excess food due to overfeeding.
They "glide" along surfaces such as the inside of the glass, ornaments and even the underside surface of the water, but are most noticed on the inside glass just above the gravel. These worms do not swim throughout the open water but can get caught up in currents and are expelled from one area to another where they will attach themselves to any surface they can...including fish. Some may have even been brought into your aquarium on a fish or two that you recently added.
They are not harmful, but unless removed through cleanings and water changes, they can accumulate to very large numbers. Should you discover these tiny white critters in your tank, give it a thorough cleaning and cut back on the amount of food you feed your fish. Eventually, any that may not have been removed through your cleaning, will die off from starvation.
Another way to help eliminate them is to equip your aquarium with a bottom feeder such as a plecostomus. Not only will this fish help to keep your tank free of excess food, but it will also eat the planaria and any other uninvited guests that may be in your tank. Some fish such as tiger barbs, angelfish and most fry have also been known to eat these invaders, as you may have noticed that they sometimes "pick" at the glass or even other fish.