The brown stuff in your shot looks to be Diatoms. They are a part of what sometimes is called "New Tank Syndrome". But diatoms, are very interesting but very tiny creatures. Try and Google the term. The basic foods for them are silica based. They use the dissolved Biogenic silica to make their little skeletons out of. (there are bigger terms but biogenic suffices)..... So no dissolved silica, no Diatoms. But most any new tank will have lots dissolved silica, just from the new substrate etc.
This is one of a number of the reasons to use RO-DI water, so we do NOT keep feeding the little buggers.. My well water here has a lot of silica in it, I live near US Silica in Berkeley Springs WV. My water is full of the stuff
Heck before I started this obsession, I did not even realize that silica would dissolve!!! But I guess in millions or billions of years water has worked at doing it, most anything will dissolve.
Hope that explains what and why that brown stuff, is in your tank. It most likely will get worse in the next few days, but will disapear with time. But RO-DI water will help if you are not using it already........ Warren