I agree. I'm sure the platies are just scavenging for food, and they just happen to be scavenging directly on the pleco. I would seriously doubt that they would be able to do the plec any harm whatsoever.
Back when I had platies, the fry used to hang around very close to my clown pleco...even underneath his little log and everything. It kept them safe from the other adult fish in the tank, and he tolerated them pretty well. At feeding time, the plec would simply raise his fins and swish his tail back and forth to scare off the platies.
I was more worried about the platies being harmed than anything else. I'm sure your plec will be just fine.
Big Vine