That helps a bit, but I still need that full tank shot (FTS), a picture is worth a thousand words
Without it I can't be sure I'm giving you the right advice. Maybe take a look at this first if you haven't already
Algae
But from what you've provided so far... It looks like diatomaceous or brown algae. It's common in new setups and low light tanks. Perhaps why the leaves near the surface aren't effected.
Next time you do a water change try to rub as much off the leaves as possible, it should come off quite easily and syphon it out.
Since you have gravel you should probably start a lean dosing regime. Well fed healthy plants have a greater chance of outcompeting alage. Take a look at these articles
Non Co2 Methods and
The Soil Substrate or Dirted Planted Tank - A How to Guide. Follow the advice. You might also want to search Darrel's
@dw1305 "Duck Weed Index". It uses floating plants as a nutrient level indicator, when they show signs of deficiency, it's time to dose.
Clean your filters and keep them clean. You may want to consider an external canister filter with 5-10 times the turnover of the tank volume per hour. This will increase the flow around the tank and help deal more efficiently with any organic waste, which encourages algae.
Not sure what your planting density is since I don't have a FTS but you might also want to consider introducing more plants. High density planting infers a great deal of biological stability and algae have a hard time competing. Also think about adding some Amano shrimp and cherry shrimp, they'll help your otos out.