At 3 weeks, you are still right where you should be. Keep feeding the ammonia a wee bit, and keep the nitrites under 5ppm. I can't really tell what the debri is, but you are going to get diatoms, which is a brown substance that brushes off easy, then after that dies, you will have some green hair algae. These are common in new tanks. Do some time studying the requirements for whatever your light level is, as far as nutrients and co2, or other carbon sources.
Standard 10 Gallon (20 x12x12).
Low tech, Sand substrate.
Eheim 2211, and Aqueon Quietflow 10.
Finnex Stingray LED, 20" above substrate
2 Anubias Nana Petite, Christmas moss.
4 Halequin Rasborahs, 5 RCS.