yes plecos with graise on algae and may eat leftovers, but they are simply not meant for cleanign up your tank their diet should still be supplemented.
But plecos do poop alot and theres realyl nothing you can do about it but clean it up alot.
I recently lost my Pleco and my water parameters changed drastically. Leveled off and tank is clearer. I was doing 2-3 water changes a week. My Pleco was about 14" big fish big poop.