Well, there's no way that I know of to "make" them eat the algae off a certain area....but it is a thought! Ha! If someone knows how to train a pleco to eat algae on command, I wanna know about it! LOL!
Yes, you do need to feed them....sinking algae wafers are good, so are fresh veggies like cucumber, squash, zuchinni.....but you will have to use something to keep the veggies submerged. I use a small piece of slate and a rubber band is used to hold the veggies to the slate.
You will also need to make sure there is a piece of driftwood in the tank too. Many species require the wood in their diet.
Do you know what kind of algae eater you have?