Algae.. and looks to be brown algae.. ick.
Live plants, keeping the tank light on too long (10+ hrs), not siphoning/cleaning enough weekly can cause algae to happen..
Brown algae usually pops up in newly established tanks before the tanks are stabilized.. roughly 2-12 weeks or so. It can disappear once the tank stabilizes, but until then avoid overfeeding and make sure to keep up on the weekly water changes.
Lessening the lighting won't deter it, as it will grow in low light and unfortunately, will out compete the green algae.
If the tank has been established, cycled and up for a long period of time, then you would need to do other things- such as more thorough substrate vacuuming, and an increase in water changes.
Even then it's pretty much essentially impossible to remove the algae.. one theory I tend to think is that if the silicate to phosphate ratio is high then diatoms are likely to have growth advantage over true algae types and cyanobacteria.
The silicate can come from tap water.. also leeched in from new glass aquariums. Possibly also from silica sand/gravel substrate to some extent.
But once the leeching is slowed and phosphate is accumulating the ratio should change in favor of the phosphate and green algae will start to grow and the brown will lessen.
Just continue to clean the tank, make sure to do siphoning weekly if you don't have live plants- live plants 1-2x a month. And keep cleaning it off.. and hope for the best.