Looks like brown diatoms. Usually you get them in new set ups, but you get them in old set ups too, especially with low lighting. Test to see if you have any ammonia in the tank, which would cause it. Also bad quality substrate or rocks that leak silicates into the water could cause it too.
I wouldn't worry too much about it. Otocinclus love to eat them. I would get a school of them If I were you. If you can't, I would deffo work on the lighting/ammonia level.