I believe their some sort of planaria. Anyway, thats besides the fact. Theyre from overfeeding and left over food. I got them several years ago, just vaccume the sand or rock etc. Really clean the substrate and aquarium. Do a large waterchange of the same temp then slowly turn up the heat, 2 to 4 degrees. You may have to do another waterchange and vacuume a week or two later. They wont harm the fish, theyre just unsightful