brown diatom algae is what you have going on. And it is a normal new tank syndrome process. You more than likely have high silicates in your tap water and this helps feed the algae. With my tap water i get this with all new set ups for the first 6-8 months and then it just clears up.
If you have fake plants in there use some algae be gone or another product designed to kill algae.
If you have live plants just give it time and keep up on the maintenance.
Etheir way it will pan out as time goes on. You could look into getting a otto that will eat diatom algae. And also extra water changes will help out as well.
You might do a search about diatom algae and get more info.