You could grow floating plants to stop the rocks getting algae on them. If you used a small plant like Duckweed, the fish would eat that as part of their diet. If you grow Water Sprite, you could plant that in the gravel when you have too much and it will grow in the gravel or floating on the surface.
Your pleco might also be eating the algae. The pleco should have some driftwood in the tank to chew on. They use the wood to help with their digestion. You could add a small bit in the back corner and it wouldn't hurt the aesthetics of the tank.
If you have fluorescent globes above the tank, they produce less light as the globes age and that will normally reduce plant/ algae growth.
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I think the easiest way to prevent disease outbreaks in an aquarium is by doing big regular water changes, gravel cleaning the substrate, and cleaning the filter on a regular basis. In addition to this, quarantining all new fish for 1 month before adding them to a display tank will also help prevent unwanted diseases getting into the main display.