Diatoms are usually small, stuck-on spots, not long stringy stuff. I think this is a different group of algae.
Keep on removing as much of it by hand as you can.
Do not use H2O2, Excel or any other chemical method of control while the tank is still cycling. You could try altering the fertilizer or the CO2. I am not sure if that will be the final answer, though. The nitrogen is not the same while the tank is cycling, so I am sure conditions will be different once the ammonia and nitrite are under control. This stuff still grows in a cycled tank.
Maybe it is rhizoclonium.
Hopefully the livestock will be interested in it, when it is time to add them.