They are comon problem in a fish tank. Those little white worms wont harn your fish or do anything to your water. It is caused by overfeeding. often found in a flowerhorn tank. How flowerhorn eats, they take the pellet in their mouth then little pieces of it comes out of its gills. I guess after he grinds them up. But those little pieces he dosnt eat and it stays on the floor. And thats how those little white worms develop.
To get rid of it either use a uv light (works for me) or do frequent water change and add salt.