Need help What is this

[Eyemdread]

Can someone please tell me what this?
Tank has been cycling for about 3 weeks

 

[MaxxuM]

The brown powdery stuff? Diatoms. Perfectly normal and if you keep your lights off it should burn off pretty quick. You'll never be 100% rid of it though. It's always there. It's a warning sign that there are too many nutrients in the system and it's a normal stage of new aquariums.