Diatoms, very common in newer tanks. See the link below for how to best deal with it. Harmless to fish, just becomes unsightly.
Brown Diatom Algae Control | Aquariums Life
Hope this helps.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.