first off, plecos should not rely on algae to survive. if you are going to keep plecos, you need to feed them veggies and dry foods to suppliment their diet.
second, your problem is diatoms, not algae. and plecos do not eat that. this is an entirely different problem all together.
and lastly, if your whole point of getting a pleco was to get rid of algae, you will soon discover you made a terrible mistake. even if they did get rid of every bit of algae in the tank, the produce easily twice as much waste as they consume in algae. unless you increase yourbiological filtration, you are looking at a major jump in your bio-load.
oh and a side note: if its a common pleco, it will potentially outgrow your 72g (assuming thats the pleco, and tank combination you have)