Do you mean On the Glass, Inside the Tank? And it takes a few Seconds to Notice them? I have that. It's quite common. I think it's called, Planaria.
It does NOT pose a Threat to the Fish. However, there IS a reason that it's there. As a Flat Worm, they need a Food Source to begin with. Are you Overfeeding your Tank? It's a Community, so you need Food for EVERYONE.
Watch for ANY TYPE of Wastes. Food Waste, or Dead Plants. Overfeeding, is the Main Cause of this.
I too, am Decreasing Feedings.
Anyways, to get Rid of it you have to Feed your Fishes the Minimal, and have VERY CLEAN WATER. Even more Cleaner than before. Have a Thorough Cleaning. And NO Overfeeding.
Watch for Food in the Gravel.