Yes, most likely the yellow-brown you are seeing is the start of your diatom bloom. The diatom bloom is caused by the available nutrients in the tank which make the microalgae grow and multiply in an uncontrolled state, the bloom will not end until the diatoms have used up all the available nutrients. I personally introduce a small clean-up crew (usually 2 to 5 asterea turbo snails only (since I've found them to be the most effective at eating diatoms) depending on the size of your tank) as soon as I start seeing visible signs of the diatom bloom starting. They will start eating and help control the extent of the outbreak - they're not going to prevent it from happening, because you are getting thousands of diatoms a day - and not even the snails constant eating can keep up with that reproductive rate.
Check out other threads on the same topic for other peoples thoughts and opinion as well.
