Are you spot-feeding the pleco? How large is s/he? Once in a while even a well-fed plec will choose to prey on tankmates - evidently other fishes' slime coat tastes good - but most often plecs who do this are starving. My rhino is still a baby, just 3.5" long, and can put away a Hikari algae wafer in a surprisingly short time! His appetite rivals that of my full-grown male bristlenose, who is longer and much heavier-bodied.
If you're not spot-feeding, try giving your rhino thawed frozen veggies like zuchinni (courgette), green beans, spinach, peas etc. As well, you can give him/her bottom-feeder wafers, algae wafers and occasionally meaty foods like shrimp.