@ombomb's idea does work. If you have a small tank (5.5gallons, for instance), you can place a number of smooth, largish rocks in it, place the whole thing in the sun, and then rotate rocks in and out of your main aquarium. If you stagger the rocks correctly you can put a new one inside your main aquarium every day, or every few days, depending on how many rocks you have. You could also use small ceramics as @ombomb noted, pieces of immersible wood, etc.
Mostly what you'll get is regular green algae that a pleco will eat. If you get other types of algae, you simply have to wash the tank out or bleach it, clean the substrates off, and then start over.
Depending on the size of the substrates, the pleco will enjoy a small snack or a larger meal. I don't know for certain if they will enjoy the live algae more or less (I suspect they do), but if it gives the owner pleasure to see the pleco out and about and eating real algae, then what's the harm in that?