The short answer
Algae on plant leaves almost always means the tank is out of balance — most often too much light for the nutrients and CO2 available. Healthy, fast-growing plants outcompete algae for light and food; weak, starved or slow plants leave the door open. The fix is to rebalance, not to scrub harder: dial back light, feed the plants, and keep the tank stable.
The usual cause: too much light
Light is the accelerator. Give the tank more light than the plants can use — because nutrients or CO2 run short — and the surplus energy goes to algae instead. Start here: put the light on a timer for 6–8 hours, and if it’s an intense fixture over a low-tech tank, reduce the intensity or duration. See how much light plants really need and how long to leave the light on.
Feed the plants so they win
It sounds backwards, but starved plants make algae worse. A plant that can’t grow can’t compete. A steady all-in-one fertiliser keeps plants growing and mopping up the nutrients algae would otherwise use. In a high-light tank, add CO2 so plants can actually use that light.
Clean-up and back-up
Remove badly-coated leaves, do a couple of water changes, and add algae-grazers like Amano shrimp, otocinclus or nerite snails to help keep new growth clean. For specific types see brown algae, black beard algae and our algae removal guide.