Not sure if it's some sort of phytoalgae, or a fungus, or what. It's eating holes in the Java fern leaf in the center of the pic, and it also attacks the Anubias. I think it's an algae. Anyway, is there a way to get rid of whatever this is?
Classic symptom of potassium deficiency. The color of the leaves also indicates some other nutrient deficiencies.
I'd recommend buying some dry fertilizer to dose your tank with.