What a pleco will actually do is eat plant matter in order to grow. Because they survive mainly n plant matter, which is low in nutrients compared to animal matter, they will excrete more waste than a meat eater. It is true of all plant eaters and is not really a problem except for aesthetics. Depending on the particular algae, a pleco may or may not eat it. The real answer to algae is twofold. First do what you can to minimize the growth and second remove the algae when you clean the tank. Adding a fish to do your work is always a losing proposition. Although they may remove the algae, they will cause you other maintenance issues.
If you like a pleco, get it and enjoy it for itself. If there are fish you would rather have, clean your own tank and get the fish you really like.