Well at that size, it has to have survived on something other than algae for this long. Where did you pick it up from? Perhaps you can see what it was eating before. I'd test your water param's, too, just to be on the safe side (although you'd really have to eff up your water to harm a pleco this mature).
To WyldFya's point, mango is an easy way to coax them into eating. It's like crack for omnivorous plec's.